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