Switch to a slimmer distroless base image.

At a high level, it switches us to a distroless base container image, but that also includes several related bits:

- Add a writable /tmp but make the rest of our filesystems read-only at runtime.

- Condense our main server binaries into a single pinniped-server binary. This saves a bunch of space in
  the image due to duplicated library code. The correct behavior is dispatched based on `os.Args[0]`, and
  the `pinniped-server` binary is symlinked to `pinniped-concierge` and `pinniped-supervisor`.

- Strip debug symbols from our binaries. These aren't really useful in a distroless image anyway and all the
  normal stuff you'd expect to work, such as stack traces, still does.

- Add a separate `pinniped-concierge-kube-cert-agent` binary with "sleep" and "print" functionality instead of
  using builtin /bin/sleep and /bin/cat for the kube-cert-agent. This is split from the main server binary
  because the loading/init time of the main server binary was too large for the tiny resource footprint we
  established in our kube-cert-agent PodSpec. Using a separate binary eliminates this issue and the extra
  binary adds only around 1.5MiB of image size.

- Switch the kube-cert-agent code to use a JSON `{"tls.crt": "<b64 cert>", "tls.key": "<b64 key>"}` format.
  This is more robust to unexpected input formatting than the old code, which simply concatenated the files
  with some extra newlines and split on whitespace.

- Update integration tests that made now-invalid assumptions about the `pinniped-server` image.

Signed-off-by: Matt Moyer <moyerm@vmware.com>

deploy/supervisor/deployment.yaml

@@ -65,6 +65,7 @@ spec:
       labels: #@ defaultLabel()
     spec:
       securityContext:
+        readOnlyRootFilesystem: true
         runAsUser: #@ data.values.run_as_user
         runAsGroup: #@ data.values.run_as_group
       serviceAccountName: #@ defaultResourceName()
@@ -80,9 +81,8 @@ spec:
           image: #@ data.values.image_repo + ":" + data.values.image_tag
           #@ end
           imagePullPolicy: IfNotPresent
-          command: #! override the default entrypoint
-            - /usr/local/bin/pinniped-supervisor
-          args:
+          command:
+            - pinniped-supervisor
             - /etc/podinfo
             - /etc/config/pinniped.yaml
           resources:
@@ -131,9 +131,11 @@ spec:
             failureThreshold: 3
       volumes:
         - name: config-volume
+          readOnly: true
           configMap:
             name: #@ defaultResourceNameWithSuffix("static-config")
         - name: podinfo
+          readOnly: true
           downwardAPI:
             items:
               - path: "labels"