Alter execute_alembic_command() to not assume all commands

Alter execute_alembic_command() to not assume all commands

Alembic 0.7.1 is adding new diff directives add_fk and remove_fk.
The logic in execute_alembic_command() appears to assume all potential
commands are present in METHODS.   This patch adjusts it so that
it checks METHODS first, allowing the tests in neutron.tests.unit.db.test_migration
to pass.  However, if the design here requires that all potential
Alembic commands are handled, then this would not be the fix,
though I recommend that the potential list of Alembic directives
not be hardcoded.

Change-Id: Ib08685be252e1fdb0f10853f8fc9a7e19ac54a94
Closes-Bug: #1397796