make ssh_authorized_key readonly

This is a rather bold and naive move to just plainly fix puppetlabs#92. It makes
all authorized_keys  generated by this module to be readonly, publicly
readable. This might break some things, and is untested.

A proper solution would probably be to hook into a File resource there
that could be overriden properly.

Fundamentally, the problem here is that we are managing multiple
resources that hit the same actual file on disk: ideally, we'd have a
mode parameter to the resource here, but then we could get into
conflicts if multiple invocations of ssh_authorized_key use different
mode parameters.

lib/puppet/provider/ssh_authorized_key/parsed.rb

@@ -39,7 +39,7 @@ def dir_perm
   end
 
   def file_perm
-    0o600
+    0o444
   end
 
   def group_writable_perm