Fix snapshot-list filter key 'usage'

Manilaclient sends filter key 'usage' with snapshot-list request anytime.
But it is incompatible with juno version of Manila.
Make manilaclient avoid sending this key in case it is not specified
explicitly.

Change-Id: Iada90df5b88848cb4cdb78d1a550e726b8829428
Closes-Bug: #1394974
Merge "Fix snapshot-list filter key 'usage'"