One volume type can point to multiple backend driver

Problem Statement:
Currently, the OpenStack cinder volume type does not allow to have more than one volume_backend_name.
Ex:
Volume type iscsi-san01 with volume_backend_name = Netapp-01
Volume type iscsi-san02 with volume_backend_name = Netapp-02
It restricts the expansion of cloud platforms in case of infrastructure has many backends that need to create separate volume types, which leads to many changes in end-user resource allocation and confusion in choosing from many volume types.

Proposal:
One volume type can have more than one volume_backend_name and can be defined by:
volume_backend_name = Netapp-01,Netapp-02
And cinder-scheduler can random from these options to determine where new volume placement

Usecase:
With QoS limit purpose
Before:
Volume type: SSD_2000_1 (limit 2000 iops) with volume_backend_name=Netapp-01
Volume type: SSD_2000_2 (limit 2000 iops) with volume_backend_name=Netapp-02

After this change:
Volume type: SSD_2000 (limit 2000 iops) with volume_backend_name=Netapp-01,Netapp-02

So end-user can see one volume type throughout, although the infrastructure has many backends device