creates the migration user, defaults the maintenance user passwords to
null
adding conditional for mongo read_only user
use xqueue application user for migrations when
COMMON_MYSQL_MIGRATE_PASS is not set
using edxapp_mysql_user_migrate user to run migrations
adding INDEX permission to migrate user
updating create_dbs to give the right perms to the migrate user
Merge pull request #934 from edx/jarv/add-migrate-user

creates the migration user, defaults the maintenance user passwords to