fixes #4660 - stub omshell during DHCP tests

It looks like omcmd/shell wasn't actually stubbed out, so we were probably making real calls to a DNS server at 192.168.122.1.  Extended tests for the SPARC attributes while I was here, so we're checking more than the response code.

Three green runs:
http://ci.theforeman.org/job/test_proxy_develop_pull_request/236/
http://ci.theforeman.org/job/test_proxy_develop_pull_request/237/
http://ci.theforeman.org/job/test_proxy_develop_pull_request/238/