Generic: add service instance mgr set up method
Generic: add service instance mgr set up method

This patch adds a method which is responsible of setting up
the service instance manager for the Generic driver.

The purpose is letting classes inheriting the Generic Driver class
easily use other service instance managers by overriding this method.

The soon to be introduced Windows Share Driver will benefit from
this change.

Change-Id: I1eab7210765adc2a131a1f850cca6f6612ec58cb
Partially-implements: blueprint windows-smb-support

NetApp/manila,scality/manila,jcsp/manila,redhat-openstack/manila,bswartz/manila,vponomaryov/manila,redhat-openstack/manila,openstack/manila,bswartz/manila,jcsp/manila,scality/manila,vponomaryov/manila,sajuptpm/manila,openstack/manila,NetApp/manila,sajuptpm/manila