EMC VNX Cinder Driver Update

EMC VNX Cinder Driver Update

VNX Direct Driver was contributed in Icehouse and updated in Juno.
This commit is to continuously improve the driver with the
following enhancements in Kilo:

* Performance improvement, especially the synchronized operations
initiatlize_connetion and terminate_connection.
* LUN Number Threshold Support
* Initiator Auto Deregistration
* Force Deleting LUN in Storage Groups
* Code refactor to enhance the robustness

Change-Id: Id263a5d0405ba942582ce06beed09b436b80ff3c
Implements: blueprint emc-vnx-direct-driver-kilo-update