Sync oslo-incubator code

Sync code to commit: b7ad6ddab8b1d61bf4f52ccaa461a9d68809747b

The oslo db API have been changed, so sqlalchemy driver have been updated
to match these changes.
We now use sqlalchemy/migration.py from oslo.db instead of the
ceilometer one.
The unused alembic code have been removed, because it is handled by
oslo.db now.

Closes bug: #1293409

Change-Id: I480d678a7165f32b92c5c9b4c55491c0edbc0991