R1:06b0f0d8abb1

Pass URI to Elastic engine from outside

Summary:
We need to generate the URI dynamically.
This code is also generally better.

Test Plan: Created custom search selector passing the custom URI to engine.

Reviewers: epriestley, edward

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2655