Volume should be allowed to be associated to multiple generic volume groups

Problem Statement:
Current Openstack does not allow a volume to be part of multiple volume groups. Which restricts some of the important features like multiple VM snapshots, Remote copy replication etc.
Most of the storage backends allow volume to be part of multiple consistency groups based on the group-type selected.

Proposal:
Allow the volume to be part of multiple volume groups.

Use Cases:
- Allow taking multiple consistent snapshots for alll or subset of volumes that are attached to a VM.
  Timestamp1 (vol1, vol2): Create the group and then group snapshot.
  Timestamp2:(vol1, vol2, vol3) Not allowed: You cannot take another snapshot since volumes are already part of another CG

- This problem becomes more obvious when replication groups are used, In most of the storages there are 2 types of consistency groups
1. remote replication CG which is used for HA/DR
2. snapshot enabled CG, which is used for copy services.
In this case if volume is added to remote CG to support DR. Then user cannot perform snapshot operations on this as they cannot add those volumes to another volume group for copy operations.