Re-run config validation from bin/search

Summary:
Ref T12450. Normally, we validate config when:

• You restart the webserver.
• You edit it with bin/config set ....
• You edit it with the web UI.

However, you can also change config by editing local.json, some_env.conf.php, a SiteConfig class, etc. In these cases, you may miss config warnings.

Explicitly re-run search config checks from bin/search, similar to the additional database checks we run from bin/storage, to try to produce a better error message if the user has made a configuration error.

Test Plan:

$ ./bin/search init
Usage Exception: Setting "cluster.search" is misconfigured: Invalid search engine type: elastic. Valid types are: elasticsearch, mysql.

Reviewers: chad, 20after4

Reviewed By: 20after4

Maniphest Tasks: T12450

Differential Revision: https://secure.phabricator.com/D17574