docs: Update upgrade guide for 0.16

        - Include introduction section
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!
pithos: Fix latest_version_list query

Fix query in latest_version_list due to wrong
commit merge.
pithos: Add test

Add test for listing only objects that do not have specific metadata.
pithos: Several PEP8 and test fixes
pithos: Backend refactoring

Remove 'pithos.backends.base' module.
Define backend exceptions in a separate module.
pithos: Provide descriptions for backend errors
pithos: Handle backend errors centrally
pithos: Cleanup 'pithos.api.util'

Use functools.partial to reduce code.