MCS service account authentication with Mkube (#166)

`MCS` will authenticate against `Mkube`using bearer tokens via HTTP
`Authorization` header. The user will provide this token once
in the login form, MCS will validate it against Mkube (list tenants) and
if valid will generate and return a new MCS sessions
with encrypted claims (the user Service account token will be inside the
JWT in the data field)

Kubernetes

The provided `JWT token` corresponds to the `Kubernetes service account`
that `Mkube` will use to run tasks on behalf of the
user, ie: list, create, edit, delete tenants, storage class, etc.

Development

If you are running mcs in your local environment and wish to make
request to `Mkube` you can set `MCS_M3_HOSTNAME`, if
the environment variable is not present by default `MCS` will use
`"http://m3:8787"`, additionally you will need to set the
`MCS_MKUBE_ADMIN_ONLY=on` variable to make MCS display the Mkube UI

Extract the Service account token and use it with MCS

For local development you can use the jwt associated to the `m3-sa`
service account, you can get the token running
the following command in your terminal:

```
kubectl get secret $(kubectl get serviceaccount m3-sa -o
jsonpath="{.secrets[0].name}") -o jsonpath="{.data.token}" | base64
--decode
```

Then run the mcs server

```
MCS_M3_HOSTNAME=http://localhost:8787 MCS_MKUBE_ADMIN_ONLY=on ./mcs
server
```

Self-signed certificates and Custom certificate authority for Mkube

If Mkube uses TLS with a self-signed certificate, or a certificate
issued by a custom certificate authority you can add those
certificates usinng the `MCS_M3_SERVER_TLS_CA_CERTIFICATE` env variable

````
MCS_M3_SERVER_TLS_CA_CERTIFICATE=cert1.pem,cert2.pem,cert3.pem ./mcs
server
````

restapi/embedded_spec.go

@@ -754,6 +754,40 @@ func init() {
         }
       }
     },
+    "/login/mkube": {
+      "post": {
+        "security": [],
+        "tags": [
+          "UserAPI"
+        ],
+        "summary": "Login to Mkube.",
+        "operationId": "LoginMkube",
+        "parameters": [
+          {
+            "name": "body",
+            "in": "body",
+            "required": true,
+            "schema": {
+              "$ref": "#/definitions/loginMkubeRequest"
+            }
+          }
+        ],
+        "responses": {
+          "201": {
+            "description": "A successful login.",
+            "schema": {
+              "$ref": "#/definitions/loginResponse"
+            }
+          },
+          "default": {
+            "description": "Generic error response.",
+            "schema": {
+              "$ref": "#/definitions/error"
+            }
+          }
+        }
+      }
+    },
     "/login/oauth2/auth": {
       "post": {
         "security": [],
@@ -1719,7 +1753,8 @@ func init() {
           "type": "string",
           "enum": [
             "form",
-            "redirect"
+            "redirect",
+            "service-account"
           ]
         },
         "redirect": {
@@ -1727,6 +1762,17 @@ func init() {
         }
       }
     },
+    "loginMkubeRequest": {
+      "type": "object",
+      "required": [
+        "jwt"
+      ],
+      "properties": {
+        "jwt": {
+          "type": "string"
+        }
+      }
+    },
     "loginOauth2AuthRequest": {
       "type": "object",
       "required": [
@@ -2869,6 +2915,40 @@ func init() {
         }
       }
     },
+    "/login/mkube": {
+      "post": {
+        "security": [],
+        "tags": [
+          "UserAPI"
+        ],
+        "summary": "Login to Mkube.",
+        "operationId": "LoginMkube",
+        "parameters": [
+          {
+            "name": "body",
+            "in": "body",
+            "required": true,
+            "schema": {
+              "$ref": "#/definitions/loginMkubeRequest"
+            }
+          }
+        ],
+        "responses": {
+          "201": {
+            "description": "A successful login.",
+            "schema": {
+              "$ref": "#/definitions/loginResponse"
+            }
+          },
+          "default": {
+            "description": "Generic error response.",
+            "schema": {
+              "$ref": "#/definitions/error"
+            }
+          }
+        }
+      }
+    },
     "/login/oauth2/auth": {
       "post": {
         "security": [],
@@ -3834,7 +3914,8 @@ func init() {
           "type": "string",
           "enum": [
             "form",
-            "redirect"
+            "redirect",
+            "service-account"
           ]
         },
         "redirect": {
@@ -3842,6 +3923,17 @@ func init() {
         }
       }
     },
+    "loginMkubeRequest": {
+      "type": "object",
+      "required": [
+        "jwt"
+      ],
+      "properties": {
+        "jwt": {
+          "type": "string"
+        }
+      }
+    },
     "loginOauth2AuthRequest": {
       "type": "object",
       "required": [