Deprecate sqlite in favor of sqlalchemy

Since the default engine used by the new sqlalchemy driver is sqlite, it
doesn't make sense to keep the sqlite driver around. This patch replaces
all usages of `sqlite` with `sqlalchemy`.

Although sqlite is being deprecated, the patch keeps the entry point for
backwards compatibility so that environments currently using the sqlite
backend won't be suddenly broken.

Implement blueprint: sql-storage-driver

Change-Id: Ib7e32fa56ab0c6621dc9a888feb6b0e4981b4e91