Support for independent alembic branches in sub-projects

Support for independent alembic branches in sub-projects

Sub-projects shall now register their independent alembic migrations
via entrypoints in setup.cfg, and neutron-db-manage will discover them
and run them automatically.

If a service or sub-project is specified explicitly, then
neutron-db-manage will run on only that service or sub-project.

The advanced services project are just special cases of sub-projects.
For example, specifying the CLI option '--service lbaas' is the same
as specifying '--subproject neutron-lbaas'.

Specifying no service or sub-project will cause neutron-db-manage to
run the command on neutron and all installed sub-projects.

Added and consolidated documentation into devref for alembic migrations.

Partial-Bug: #1471333
Partial-Bug: #1470625

Change-Id: I9a06de64ce35675af28adf819de6f22dc832390d