bash completion for new volcli policy-related commands

[dseevr]

@vvb Erik told me this is your thing :^)

New commands introduced in #367:

• volcli policy history list $policy_name
• volcli policy history get $policy_name $revision
• volcli policy watch

Example output below:

$ volcli policy upload foo < /testdata/ceph/policy1.json
$ volcli policy history list foo
1471302683
$ volcli policy history get foo 1471302683
{"name":"foo","create":{"size":"10MB","filesystem":""},"runtime":{"snapshots":true,"snapshot":{"frequency":"30m","keep":20},"rate-limit":{"write-bps":0,"read-bps":0}},"driver":{"pool":"rbd"},"filesystems":{"ext4":"mkfs.ext4 -m0 %"},"backends":{"crud":"ceph","mount":"ceph","snapshot":"ceph"}}
volcli policy watch # <-- this has no output until a policy is uploaded