Mattermost -> Elasticsearch connection broken by STRICT mTLS

When STRICT mTLS is enabled for logging (elastic/kibana), an in cluster Mattermost will fail to connect for indexing.

This can be worked around for now by setting mTLS to PERMISSIVE for logging:

logging:
  values:
    istio:
      mtls:
        mode: PERMISSIVE

We should likely add an exception to allow this...