astakos: Remove --all option from user-modify

'--all' was introduced in order to update user quota in bulk; this
functionality has now moved to project-modify. Also remove related
'--exclude' option.
astakos: Rename base to system projects

Rename base projects to system projects in management commands and API.
Update docs and changelog.
pithos: Fix KeyError raised by the backend

If not an external quota component (eg astakos) is used the Pithos+ backend
enforces the quota restrictions.
However, if no quota limit is set for an account, it expects a default quota
policy to be set.
This commit fixes this issue.
pithos: Implement 'delete_by_uuid' backend method.

Extend the Pithos+ backend by providing a method that gets as input the
object's UUID and deletes the object.
pithos: Provide tests for 'delete_by_uuid' method.
Merge branch 'feature-pithos-delete-by-uuid' of https://github.com/papagian/synnefo into develop
Mention need for unique UUIDs across projects/users
docs: Correction on snf-ci's configuration file

The configuration file for snf-ci has erroneously been named as
'ci-wheezy.conf'. Change it to 'ci_wheezy.conf'.