Clusters API Implementation

adds clusters api, with mongodb sharding as the first implemenation.

Co-Authored-By: Michael Yu <michayu@ebaysf.com>
Co-Authored-By: Mat Lowery <mlowery@ebaysf.com>
Co-Authored-By: rumale <rumale@ebaysf.com>
Co-Authored-By: Timothy He <the@ebaysf.com>

Partially implements: blueprint clustering

Change-Id: Icab6fc3baab72e97f3231eaa4476b56b8dafb2a3
Merge "Clusters API Implementation"