archipelago: Adjust permissions of directories

Make all Archipelago directories, files and fifos accessible by the group
Archipelago is configured to run as. This way all Archipelago components can
communicate and signal each other independently from their effective user id.
archipelago: Move cephxid to Radosd class
mapper: Fix mapper log message
archipelago: Fix None value for v0_size
conf: Change the default user/group to archipelago
Minor python fixes
python: Return a more descriptive message on xseg_join failure

If xseg_join fail, raise an Error with a more descriptive message based on
errno.
filed: Create dirs with RW permissions for group
peer: Always use posixfd peer type