Reload TLS certificate in the process manager

[sebgl]

TLS certificate are mounted into Elasticsearch pods through a secret volumes, and used by the process-manager.
When a certificate is close to expire, the operator signs a new one, propagated through the secret volume. Elasticsearch reloads it automatically when a change is detected on disk.

Since we don't do the same in the process-manager, we keep on using an old outdated certificate. The operator detects an expired certificate when issuing requests to the process-manager and returns an error, preventing restarts to happen correctly.

[thbkrkr]

Closing as we decided to retire the process manager see #1199.