[neutron-db-manage] revision: properly bootstrap a new branch

[neutron-db-manage] revision: properly bootstrap a new branch

If the intended branch is not present, bootstrap it:

- create directory;
- mark the new migration script with proper branch label;
- make the script down_revision == None to indicate it's a new branch;

One missing thing is making the script depends_on the previous release
branch. This is currently unsupported by alembic though.

Partially-Implements: blueprint online-schema-migrations
Change-Id: Ib3b9dfcbdb56db99b07a8e54629dda5933e1c1f5