Set required umask for snf-manage commands

In order to communicate with Archipelago, Pithos backend needs to create
named pipes (under '/var/run/shm/posixfd') that can be read/written by the
group that Archipelago is running. This is achieved by using 'setgid' in the
specified directory combined with a proper 'umask'. For Gunicorn workers, the
umask is set by gunicorn-hooks. However, snf-manage commands did not set the
needed umask which resulted in wrong permissions.

This commits includes an ugly workaround to bypass this issue by setting
the needed umask in 'SynnefoManagementUtility' for subcommands that are
handling images, snapshots and files and for subcommands that define
the 'umask' class attribute.
docs: Remove reference to PITHCAT_UMASK

From snf-image 0.16.3 the 'PITHCAT_UMASK' is not required.