Add primary KEY to migration_table

[mrcnpdlk]

For Percona XtraDB Cluster tables without primary keys are prohibited:
Uncaught PDOException: SQLSTATE[HY000]: General error: 1105 Percona-XtraDB-Cluster prohibits use of DML command on a table (my_base.migration_version) without an explicit primary key with pxc_strict_mode = ENFORCING or MASTER
What about adding the primary key on version column?
I can prepare the patch.

[byjg]

It is cool a feature and I am happy to have this patch from you.

Quick question: Are you able to create the Patch for the other Databases too?