Allow 1-element arrays for primitives in cs_group

Having only one primitive in a group is a perfectly valid use
case. For example, one might have constraints related to an arbitrary
number of primitives created with create_resources, and the simplest
way of doing that is to place them all of them in a group, and have
the constraint point to that. Disallowing the possibility of ending up
with just one primitive in the group does not make much sense.

Update the check to allow 1-element arrays. Also, remove a misleading
comment that also looks like it was copied from some other code that
talked about colocations, not groups.

Fixes issue voxpupuli#152.

lib/puppet/type/cs_group.rb

@@ -23,11 +23,10 @@ module Puppet
 
       # Have to redefine should= here so we can sort the array that is given to
       # us by the manifest.  While were checking on the class of our value we
-      # are going to go ahead and do some validation too.  The way Corosync
-      # colocation works we need to only accept two value arrays.
+      # are going to go ahead and do some validation too.
       def should=(value)
         super
-        raise Puppet::Error, "Puppet::Type::Cs_Group: primitives property must be at least a 2-element array." unless value.is_a? Array and value.length > 1
+        raise Puppet::Error, "Puppet::Type::Cs_Group: primitives property must be at least a 1-element array." unless value.is_a? Array and value.length >= 1
         @should
       end
     end