FEATURE: make existing Essex volume/snapshots available in Folsom

* major change is in _lookup_lun_for_volume method. If the volume
  of snapshot was not found by its UUID name then he will try the
  lookup by EC2 name. Works both for volumes and snapshots
* self.discovered_luns contains all LUNs (volumes and snapshots)
  I use it now to lookup snapshots, where before, only volumess
  were looked-up in that dictionary
* create_snapshot now has to add newly created snapshot into
  discovered_luns with DFM LUN refresh. This slows down
  create_snapshot considerably for all snapshots
  (even Folsom-Folsom case)
* delete and create_volume_from_snapshot now lookup by snapshot
  instead of parent volume. LUN path from lookup is used directly
  instead of constructing path from volume/qtree/snapname..