add have_ipnat_rule matcher for Solaris

I've added have_ipnat_rule matcher for Solaris.

Usage

    describe 'ipnat' do 
      it { should have_ipnat_rule('map net1 10.12.0.0/24 -> 0.0.0.0/32 portmap tcp/udp auto') }
      it { should have_ipnat_rule('map net1 10.12.0.0/24 -> 0.0.0.0/32') }
    end