Fix scheduled share creation with generic driver

Generic driver uses separate module service_instance for handling of Nova VMs.
If service m-shr starts and there are existing scheduled shares for creation,
then concurrent issue appears between method 'do_setup', that is called once
on start of service and 'ensure_server', that uses stuff from do_setup.

Main error is in following:
- "Returning exception 'GenericShareDriver' object has no attribute
  'service_instance_manager' to caller"

Changes:
- moved definition of attr 'service_instance_manager' to __init__ method.
- removed attrs '_helpers' and 'share_networks_servers' from
  ServiceInstanceManager as they are redundant.
- updated unittests according to changes.
- used direct import of 'constants' submodule for neutron api,
  to be able to access it on driver init step.
- replaced generic driver with common driver interface for inheritance
  by fake driver for unittests.

Change-Id: I0fe7f097dc03eade5e0d4dfdbda0eb49588f4dde
Closes-Bug: #1362985
Related-Bug: #1330391
Merge "Fix scheduled share creation with generic driver"