Product license verification in Console (#518)

Operator UI - Provide and store License key
- New License section in Operator UI will allow user to provide the
  license key via input form
- New License section in Operator UI will allow the user to fetch the
  license key using subnet credentials
-  Console backend has to verify provided license is valid -
   https://godoc.org/github.com/minio/minio/pkg/licverifier#example-package
-  Console backend has to store the license key in k8s secrets

Operator UI - Set license to tenant during provisioning
- Check if license key exists in k8s secret during tenant creation
- If License is present attach the license-key jwt to the new console
tenant via an environment variable

Operator UI - Set license for an existing tenant
- Tenant view will display information about the current status of the
  Tenant License
- If Tenant doesn't have a License then Operator-UI will allow to attach
new license by clicking the Add License button
- Console backend will extract the license from the k8s secret and save
the license-key jwt in the tenant console environment variable and
redeploy

restapi/config.go

@@ -220,6 +220,11 @@ func getSecureExpectCTHeader() string {
 	return env.Get(ConsoleSecureExpectCTHeader, "")
 }
 
+// GetSubnetLicense returns the current subnet jwt license
+func GetSubnetLicense() string {
+	return env.Get(ConsoleSubnetLicense, "")
+}
+
 var (
 	// GlobalRootCAs is CA root certificates, a nil value means system certs pool will be used
 	GlobalRootCAs *x509.CertPool