Bug 1074632 - RFE: Manage storage node snapshots

Added "Take Snapshot" operation to StorageNode resource. This operation
takes several other parameters and it can either move or delete older
snapshots. Basically it allows to
 - keep N latest snapshots and move/delete older ones
 - keep snapshots not older than N days and move/delete the older ones
Merge pull request #95 from lzoubek/bugs/1074632

Bug 1074632 - RFE: Manage storage node snapshots