Bug 1074632 - RFE: Manage storage node snapshots

2nd piece of impl above BZ. This patch adds server side capability to manage
snapshots for storage cluster. This basically means that we regularly run
takeSnaphost operation with several parameters - so user can decide whether
to keep all of them, keep last or move older ones to specified location.

6 new private/readonly system settings have been introduced - those settings
can be updated only via Storage admin pages.

User can enable/disable snapshots management of storage cluster and set cron
expression to run management task regulary (additional 4 settings are
basically parameters for takeSnapshot operation introduced within previous
commit for this BZ). When Cluster Setting is saved in UI we re-schedule
takeSnapshot operations on all StorageNode resources in inventory.

Snapshot related code was removed from
StorageNodeManagerBean#runClusterMaintenance()
Merge pull request #97 from lzoubek/bugs/1074632

Bug 1074632 - RFE: Manage storage node snapshots (part. 2)