EMC Manila driver

EMC specific NAS storage driver. This driver is a pluggable driver
that allows specific EMC NAS devices to be plugged-in as the underlying
backend. Use the Manila configuration variable "share_backend_name"
to specify, which backend plugins to use.

VNX and Isilon plugins will be submitted later in different patches.

partially implements: blueprint emc-driver

Change-Id: Ibe174d22a5a63693fa28998459434f9db25fb878
Merge "EMC Manila driver"