Move show_diff from agent to main

This enables show_diff also for Puppet apply - which I use very often for
testing.
If runmode is 'service' and is a debian based system, ensure that
/etc/default/puppet has START=yes
pull in new auth.conf from puppet 3.5.0
Set explicit perms, owner and group on puppet.conf

When the umask on the server is 0077, the puppet.conf file has
permissions of 0600 and the file can't be read by the foreman-proxy or
puppet since the default owner is root.  By setting the perms to world
readable, we resolve these issues.

Since puppet also defaults to the file owner on the puppet master, it's
better to explicitly set the owner and group.
Release 2.1.1