Specify raise_on_warnings=False for mysqlconnector

mysqlconnector driver in sqlalchemy <1.0 uses the wrong default value
for raise_on_warnings.  With raise_on_warnings=True, many things we
expect to be warnings become fatal (like DROP DATABASE IF EXISTS
non_existent_db).

See https://bitbucket.org/zzzeek/sqlalchemy/issue/2515 for upstream bug.

Change-Id: Ic29c9bb6c692952640e3948f5ce31290e158068f
Merge "Specify raise_on_warnings=False for mysqlconnector"