Fixing neutron-db-manage with some options other than upgrade/downgrade

Fixing neutron-db-manage with some options other than upgrade/downgrade

"mysql-engine" argument was added to upgrade and downgrade option of
neutron-db-manage.
Reference commit: http://tinyurl.com/mzepbmq

migration environment's run_migration_offline/online() gets called
even for other neutron-db-manage options as well such as current,
history, stamp, branches etc. For those options since the argument
can not be set, it throws oslo.config.cfg.NoSuchOptError.
This fix tries to catch it and set the value accordingly.

Closes-bug: #1353180

Change-Id: I044daf04216ec61245ddb51689f8e50be5666e34