stein: install python3-mysqldb for sqlalchemy dialect change
At Stein, the default mysql dialect in SQLAlchemy switched back
to MySQLDB in Ubuntu; as a result the db connection URL's stored
in the nova_api database for Nova cells are stale and need to
prefixed mysql+pymysql:// to work with the pymysql dialect.
However at this point in time there is no way to update the DB
URL for Nova Cell 0.
This fix works around this problem by installing python3-mysqldb
which provides the default dialect needed when using mysql://
prefixed DB connection URI's.
