Don't fatal in ElasticSearch setup check if no "master" database is configured

Summary:
Ref T12965. See that task for discussion, and PHI36 for context.

This sweeps the fatal under the rug by skipping it, letting things move forward for now.

Test Plan: Followed instructions in T12965, got a read-only recovery after restart instead of a fatal.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12965

Differential Revision: https://secure.phabricator.com/D18440