Use dialect rather than a particular DB API driver

_get_connect_string() is too specific when choosing a DB API driver to be
used. We shouldn't force users to use neither psycopg2 nor MySQLdb as this is
not required for any actions provided by oslo.db. Specify only the dialect to
 be used and let SQLAlchemy choose one of the DB API drivers available.

Closes-Bug: #1256835
Change-Id: I685f9eecc77f2c1f920c83c5da653f3114a83679
Merge "Use dialect rather than a particular DB API driver"