Mock Resolv.getaddress in #host_to_ip

Add an expect for Resolv.getaddress in Puppet::Util::Firewall#host_to_ip so
that the test can be run when disconnected from the net. Also isolates it
should should puppetlabs.com move to a different address.
Merge pull request #110 from dcarley/expect_resolv_getaddress

Mock Resolv.getaddress in #host_to_ip