Replace XML with JSON for N1kv REST calls

Currently the cisco n1kv plugin handles XML responses from VSM.
This change will replace the httplib2 with requests library and handle
JSON responses from the VSM.

Implements: blueprint cisco-n1kv-json-support

Change-Id: Icd32a6a2ab815ccd24ad86371e927c132e204413
Merge "Replace XML with JSON for N1kv REST calls"