With XenServer as hypervisor, the commands neutron-ovs-agent in
compute node run are actually executed in Dom0. But current Dom0
plugin doesn't allow conntrack command, this patch is to add such
support.
Also, the exitcode the commands returned in Dom0 will pass through
Dom0 to neutron to make sure the plugin is only aimed executing
commands, it doesn't deal with business scenario.
Closes-Bug: #1603400
Change-Id: I304788240bcd590ec211bca052fe64594a4e6eca