cyclades: Fix small issues with snapshots

* fix IndexError in snf-ganeti-event when processing an
  OP_INSTANCE_SNAPSHOT operation without defining --disk option.
* fix TypeError in snf-dispatcher when processing an
  OP_INSTANCE_SNAPSHOT without snapshot information.

Both issues are caused by using directly the 'gnt-instance snaspshot'
command.
gtools: Make eventd tolerate unhandled exceptions

Make 'snf-ganeti-eventd' daemon tolerate exceptions while processing
events, in order to avoid losing many Ganeti events because of a failure
to process a message of specific type.
docs: Update upgrade notes for Cyclades
astakos: Fix typo

Check that holding exists before using it!