networker skeleton

worker: networker skeleton

This RP is a skeleton for the future networker based on StringsWorker

Continue of work from: https://github.com/juju/juju/pull/86
Didn't we decide to use `auto <iface>\nsource /etc/network/interfaces.d/<iface>.conf\n` instead (you said you tested it on both trusty and precise successfully)?

Also, when creating the above file, make sure you do:
`install -D -m 644 /dev/null /etc/network/interfaces.d/<iface>.conf`
before
`printf "..." > /etc/network/interfaces.d/<iface>.conf`