Advanced services support in neutron-db-manage

Partially-Implements: blueprint services-split

Change-Id: Ib8cd77e5e777be83c44b69edc0a6d2d2ba5d2ab1
Backward compatibility for advanced services

Patch implements translation from class paths to neutron to class paths
to neutron_<adv_service>. It's achieved by defining entry point in
setup.cfg which is translated by stevedore.

There will be needed patches in advanced services tree calling
get_provider_driver_class() function before importing class.

This patch specifically fixes loading service plugins and
drivers for service plugin. Patches for agents are still needed in
neutron repo and adv services repos.

Alternative and better solution would be implementing new DriverType
to oslo.config, which will have callback to
get_provider_driver_class()-like function.

Co-Authored-By: Ihar Hrachyshka <ihrachys@redhat.com>
Change-Id: I76af175c4387326a4e5ff95c2f15d8b866dedab3
Partial-Bug: 1401895
Merge "Advanced services support in neutron-db-manage"
Merge "Backward compatibility for advanced services"