Add metadata field in group and group_snapshot tables

Storage such as IBM DS8000 needs to store some specific information about group(VG and CG) as its metadata in DB, so that it can have the abilities as below:

1. when user create a new volume and put it in an existing group(via API create_volume), it can get the metadata from group, and create volume according to it.

2. when user create group, driver can know how many resource have been used by all existing groups. so it can assign remained one to the new group.

Reason:

First of all, DS8K doesn't have CG entity, it can not fetch information of CG in the storage, it needs cinder to maintain the relationship between CG and its volumes.
But groups or consistecnygroups table doesn't have metadata field, so DS8K driver needs get metadata from volumes in group object to get groups information.

this BP will focus on the feature that adding metadata field in groups and group_snapshots tables..