Control active number of REST calls from Cisco N1kv plugin to VSM

Add a config parameter to Cisco N1kv plugin to determine the total
number of active REST calls that can be made to the VSM.

Change-Id: I66433ef5673d35badcd3adc2defa43e578d4094f
Closes-Bug: #1329099
Merge "Control active number of REST calls from Cisco N1kv plugin to VSM"