Add migration 021 set_engine_mysql_innodb

Change: I1536c9276bc807aac26849e37556b465e8b5c9eb which added
migration 018 neglected to set the mysql engine to InnoDB
when it added the image_locations table. This commit adds a
new migration to set the image_locations table to the InnoDB
engine.

Change-Id: I935065d8e1929d5df075cc5c7de81793b1dd73a1
Merge "Add migration 021 set_engine_mysql_innodb"