(MODULES-439) Work around existing rules

The firewall resource is not intended to be used with rules that are not
also managed by puppet; the behavior when doing so was undefined. This
is an attempt to make it more defined.

The behavior is that any rule added by puppet will be inserted in its
given order in relation to the other rules managed by puppet, but ahead
of any rules not managed by puppet.
Merge pull request #286 from hunner/fix_source

(MODULES-439) Work around existing rules