VMware vSphere support: Performance Mgr APIs

(*) Added a dependency on 'oslo.vmware' library, in requirements.txt.

(*) Using the vmware library, implemented a wrapper class -
VsphereOperations - for querying performance stats from VC.

(*) Wrote unit test cases for VsphereOperations.

Change-Id: I445bcdb3f8ac240d1799bfd31f8b888982d7d97f
Implements: blueprint vmware-vcenter-server
Merge "VMware vSphere support: Performance Mgr APIs"