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: Raise OSError on xseg_join if appropriate

Raise the appropriate OSError when xseg_join fails. Use the errno provided by
libc.