Implement proper Keycloak Healthcheck before connecting

[pankalog]

openremote/container/src/main/java/org/openremote/container/security/keycloak/KeycloakIdentityProvider.java

Line 323 in 622375d

protected void pingKeycloak(KeycloakResource resource) throws Exception {

Using the welcome page as a tester for if keycloak is running is not the best. Keycloak now has a healthcheck API: https://www.keycloak.org/server/health

Maybe use this to check if keycloak is healthy or not before performing any actions.

The healthcheck is already enabled in OpenRemote so no Keycloak configuration changes.