Add manage/unmanage volume support for Nimble

Add manage/unmanage volume support for Nimble

This patch adds support for volume manage/unmanage for a Nimble
ISCSI driver. It uses an additional field 'agent-type' on the
backend for distinguishing volumes managed by the Nimble driver.
While managing an existing volume it sets the agent-type to
Openstack and while unmanaging sets it back to None.

Since this field was not used in the earlier versions, in order
to be backward compatible, during the initialization of the driver,
the 'agent-type' is updated to Openstack for all the existing
Nimble volumes.

Implements: blueprint nimble-add-volume-manage-unmanage-support
Change-Id: Ia960f2a2911288409977324cfc287c216b60b589