puppet+devtest: make compute nodes reachable

puppet+devtest: make compute nodes reachable

Compute nodes run libvirt, which automatically creates a default network
which has the same address space (192.168.122.*) as the libvirt default
network on the host machine where devtest is running. This overlap
causes that when a compute node wants to send a packet to the host
machine (192.168.122.1) it gets incorrectly routed through the compute
node's own virbr0 instead of br-ex. The current solution does not seem
to be enough because libvirt gets started and creates the default
network before Puppet is triggered on compute nodes. Making sure the
libvirt default network is destroyed on the compute node fixes the
issue.

We don't have any puppet modules in OPM that would deal with libvirt
networks and it's probably not worth exploring and adding one because of
this small issue (i don't expect another use case of managing libvirt
networks directly), so i'm using an exec with proper idempotency
check.

Change-Id: Icde12aa204ed1f7fa35b0525875ce07db34dc42c
Closes-Bug: #1436822