Check Jira connectivity at startup

Make sure that Jira connectivity issues are well highlighted when
the its-jira plugin starts, by reading the default Jira URL config
and performing an health-check at the start() lifecycle listener action.

Fix a regression where an incorrect Jira URL in the gerrit.config
got unnoticed since the introduction of the multi-server support.

Bug: Issue 10639
Change-Id: I5373a9defb4c21d5f6b53407f64fb69d30c388fa