Reverse proxy subset of elasticsearch API.

It has been requested that we provide programmatic access to the
elasticsearch API in addition to the Kibana web UI. Do this by reverse
proxying http://logstash.openstack.org/elasticearch/$API_REQUEST to
http://elasticsearch.openstack.org:9200/$API_REQUEST.

The only values for $API_REQUEST that will be passed through are
_aliases, _status, and _search.

Change-Id: Ib41f6d91e2e59d493218074a67155af450ec8c93
Reviewed-on: https://review.openstack.org/33316
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins