deploy: Add PITHCAT_UMASK setting to snf-image

PITHCAT_UMASK is needed so if archipelago does not run as root.
If missing the pithcat (e.g. needed for shared-filer ext provider)
becomes really slow.

Up until now we had /etc/default/snf-image as a template with
default snf-image settings commented. These settings were never
updated and as a result were different from the ones the package is
shipped with. This patch, after package installation, moves original
/etc/default/snf-image to snf-image.orig and then overrides it with
the template that includes only the settings that deploy want to
change.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>