Log an error when clustering is enabled but the authentication keys are not configured

When xwiki.authentication.validationKey / encryptionKey are not set in xwiki.cfg, each node generates its own keys in its permanent directory. In a cluster this logs users out at every node switch, with nothing in the logs explaining why.

Proposal: when observation.remote.enabled is true and a key had to be generated, log an ERROR at startup naming the two keys to set.