Support for persistent volumes

[jaredstehler]

http://mesos.apache.org/documentation/latest/persistent-volume/

My initial thought is that we could support a similar syntax to the volume definition structure for docker containers, and auto-generate persistent volume ids based off the singularity request id.

This will need some ui changes I imagine.

[jaredstehler]

TBD: whether Shared Persistent Volumes should be considered in scope for this: http://mesos.apache.org/documentation/latest/shared-resources/

[stevenschlansker]

see #1690 for a start