require mysql binding for mysql db only

the requirement for the mysql module was introduces
with Change-Id: Ie0011102d9f4dfcd50f24afcb73072090c914011

If you do not use mysql (eg. because you prefer postgres),
the module is not usable without this patch.
The implementation is similar to puppet-keystone.

Change-Id: I42b7f75fe103d4e64d8991bcc9c8a20aef8f353d