Adds use of OSMPOL_SQL_DATABASE_URI config param to connect to DB

The OSMPOL_SQL_DATABASE_URI was not being used by POL to connect
to the database. There was a hardcoded sqlite database name.
This change should now provide capability to the user to use any
SQL database engine supported by the peewee ORM (mysql/mariadb,
postgres, sqlite).

Signed-off-by: Benjamin Diaz <bdiaz@whitestack.com>
Change-Id: Iff406862fb3dfeb4453fb3213c94db6d62385d96