feature: Set scheme for probes and serviceMonitor

[TheBigLee]

The new health endpoints used by the three probes (liveness, readiness
and startup) as well as the metrics endpoint are now served on a
dedicated interface with its own port. The scheme of the interface only
servers HTTP or HTTPS. If the main keycloak interface serves HTTPS,
the internal management interface will also serve HTTPS, regardless if
the main keycloak interface servers plain HTTP or not.

It's therefore impossible for the probes or the serviceMonitor to work
with a plain HTTP scheme, if keycloak has HTTPS enabled.

We therefore need a way to specify the scheme of the internalPort via
helm values to be able to set a custom scheme for the probes and the
serviceMonitor

This fixes #778 and #781

[mruzicka]

@TheBigLee , the scheme should not be hardcoded, it would break the probes for everyone (us included) who runs Keycloak on http (behind a SSL terminating proxy) by providing these values to the chart:

proxy:
  enabled: true
  mode: xforwarded
  http:
    enabled: true

I.e. the scheme should be conditional on the .Values.enabled and .Values.http.enabled values.

Edit: It turns out it is enough to set .Values.http.enabled to true, for Keycloak to be run on http.

[TheBigLee]

@TheBigLee , the scheme should not be hardcoded, it would break the probes for everyone (us included) who runs Keycloak on http (behind a SSL terminating proxy) by providing these values to the chart:

proxy:
  enabled: true
  mode: xforwarded
  http:
    enabled: true

I.e. the scheme should be conditional on the .Values.enabled and .Values.http.enabled values.

Edit: It turns out it is enough to set .Values.http.enabled to true, for Keycloak to be run on http.

It's actually more complicated than that.
Keycloak itself can run on http, https or both. The management interface however, can only run on http or https. If Keycloak itself is running on https than the management interface will run on https as well.
The chart itself currently doesn't support setting up HTTPS directly for Keycloak. You have to set the KC_HTTPS_CERTIFICATE_FILE env variable via the extraEnv chart variable and load the certificate and key using the extraVolumes chart variable.

So there is actually no easy way to automatically determine the correct scheme for the probes.
I've refactored the code and defined a new variable http.internalScheme that can easily be overridden in the chart. Hopefully this approach is acceptable.

Edit: I found out that the serviceMonitor also needs the scheme set to https if the internal management interface is served via https. I've amended the code accordingly.