Add validation for declared volume claim templates

[pebrc]

Partially addresses #4525

Adds a validation to check that declared volume claim templates are actually mounted in any of the containers. This validation does not apply if the user configures the documented default data volume elasticsearch-data for which ECK manages the volume mount.

There is still a possibility for misconfiguration. As we are currently supporting custom volume mounts and custom data.path settings in Elasticsearch it is quite hard to fully validate the configuration. With the proposed validation it would still be possible for a user to configure a custom volume claim template, mount it in a side car and have no persistent Elasticsearch data volume. This could be addressed in two ways:

• also parse the Elasticsearch configuration during validation and make sure each data.path has a volume mount
• always configure a default data volume as we had it pre-1.4.0

[sebgl]

LGTM. Left a few comments for minor improvements.

One concern is that existing users with misconfigured volumes would suddenly have their cluster reconciliation error-out when they upgrade the operator to a version with that validation. I think that's OK (better point out the problem and stop reconciling rather than silently ignoring it?).

With the proposed validation it would still be possible for a user to configure a custom volume claim template, mount it in a side car and have no persistent Elasticsearch data volume. This could be addressed in two ways

I think I'm fine with not addressing this edge case.

[pebrc]

Jenkins test this please