nova-agent.py: add a resetnetwork command

Add an agent feature to make it easier to test out network
configuration and make it possible to use nova-agent on systems where
the root filesystem might be non-persistant (since the dom0 doesn't
give a resetnetwork command on every boot).

Tested on: Ubuntu 12.04
nova-agent.py: check length of sys.argv for resetnetwork

Avoid crashing by checking sys.argv
Merge pull request #20 from coreos/resetnetwork-command

nova-agent.py: add a resetnetwork command