Improve snapshots list API filtering

1) Added new params for filtering by:
- source share
- usage of snapshot*
- limit and offset a la pagination
- direction (asc, desc)*
- key of snapshot*

* requires server-side update

2) Added aliases for params, mostly in perspective to avoid mess of underscore
and dash symbols.

Implements blueprint improve-snapshot-list-filtering

Change-Id: I3727911f179ea01dd4cb68149fa7f4d79ca75a5c
Merge "Improve snapshots list API filtering"