We used to support subprojects with a single alembic branch.
Now the plan is to switch to a single alembic env for Neutron
and all installed subprojects. In a single alembic env, all
subprojects must use split (contract and expand) alembic branches.
The first step of removing single branch support was done in [1]
where we removed support for single HEAD files.
The --split_branches option was used by developers only. By removing
it we do not break any subproject, but force them to use split
branches for any new migration scripts.
[1] https://review.openstack.org/295905
Partial-Bug: #1497830
Change-Id: I62a191e1d22a1af4061c25fdc4a8df965d585419