pithos: Refactor statistics subquery

Use CTE to get current population.

Add partial index to versions table.
pithos: Add missing setting from conf
pithos: Fix issues with groups

* Apply checks for group name/member length
* Do not try to insert an existing member to a group (integrity error)
pithos: pep8 fixes

closes grnet/synnefo#172
Update Cyclades section in Changelog
Fix typo
Bump version to 0.16next