Set umask

Ensure we have a known-good umask.  Otherwise files such as
/etc/polkit-1/rules.d/50-libvirt-$STACK_USER.rules may not be readable
by non-root users afterwards.

Also reworded some comments to be more clear.

Change-Id: I7653d4eee062cf32df22aa158da6269b1aa9a558
Closes-Bug: #1265195
Merge "Set umask"