Ability to specify which volumes to snapshot (or not)

[ncdc]

Describe the solution you'd like
The --snapshot-volumes flag only enables or disables the snapshots, but I want to specify which ones to snapshot.

Anything else you would like to add:
There are some persistent volumes we stand up like RabbitMQ and Redis which we are interested in doing a configuration backup but are not interested in snapshotting those volumes because the data in those volumes is ephemeral. However, we do have data stores like MongoDB in which we are interested in doing a config backup AND a snapshot of the data. It would be useful to specify which pvs ark should make a snapshot of.

(from https://kubernetes.slack.com/archives/C6VCGP4MT/p1539287294000100)