Pass volume_id in request_spec for manage_existing

Pass volume_id in request_spec for manage_existing

In order to _set_volume_state_and_notify in scheduler.manager to work
correctly and error out volume on failure we need to pass volume_id in
the request_spec from c-api to c-sch. This commit implements that and
adds check for it in the unit tests for manage_existing flow.

Apart from that it removes unnecessary test that was setting volume to
MagicMock(return_value=None), which isn't actually testing anything
(volume() will return None, but we're never doing calls on volume
object).

Closes-Bug: 1526771
Change-Id: I93e16e281e952433870077309547e0ce09356a60