Add migration method to test sqldump files

This is very useful for testing real migration issues from a mysqldump.
You use it like this:

    def _pre_upgrade_031(self, engine):
       _load_mysql_dump_file(engine, 'dump.sql')

Change-Id: I0d5680d42d608bdc1cb70ccdf19a4c867b372351
Merge "Add migration method to test sqldump files"