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
add systemd service unit
switch archlinux to the systemd unit

default to using the .service unit if systemd is the init system on the
server
add netctl for archlinux

netcfg is moved to the aur, this adds netctl support, and also resolves
the use of systemd to if /sbin/init is symlinked to the symlink binary.