Fix db management: don't autocreate db on start

We should use sahara-db-manage for db creation instead of auto created
db schema from metadata. This approach was applied by adding support for
alembic-based migrations and db autocreation code was removed in
I14a0602bb34bf09086428766cb681345191436fe, but it one more place where
we're autocreating db was missed, so, it's a fix for this issue.

Additionally, cleanup custom hacking check for sahara.db calls.
Docs already contains notes about need for using sahara-db-manage for
database creation.

Note: The main driver in fixing this right now is that if a user create
database from metadata, he will not be able to migrate from Icehouse to
Juno. Only databases created by alembic could be migrated to the later
version.

Closes-Bug: #1300190
Change-Id: I991aa3402f658999735ad9607544251a6ef0e122
Merge "Fix db management: don't autocreate db on start"