EMC VNX backend support

Add support for EMC VNX direct driver.

- Create the volume driver for EMC VNX direct driver
- Create the backend for volume type support (multi-backend)
- Unit test for basic parameters

Official documentation used:
http://docs.openstack.org/trunk/config-reference/content/emc-vnx-direct-driver.html

Change-Id: Iccaea371751aac92b6d0df2011e18fd9bd6b98e9