Enhance configuration of admin credentials for actuator access

[dturanski]

Depends on #478

Currently, the configuration for src/kubernetes/skipper/skipper-config-<binder>.yaml looks like

data:
  application.yaml: |-
    spring:
      cloud:
        skipper:
          server:
            platform:
              kubernetes:
                accounts:
                  default:
                    appAdmin:
                      user: ${app-admin-user}
                      password: ${app-admin-password}
...

These come from a secretKey ref. Like

apiVersion: v1
kind: Secret
metadata:
  name: skipper
  labels:
    app: skipper
data:
  app-admin-password: cGFzc3dvcmQ=
  app-admin-user: YWN0dWF0b3I=

The deployer needs to pass these creds to the ActuatorTemplate as currently implemented. We maybe can add something like adminSecretName to the k8s deployer properties and use that to configure the app container instead of passing the creds as args.

[markpollack]

This is an experimental feature request, valid but lower priority for us to address. Look post 2.10 SCDF release.