Adds support for multiple network interfaces

Previously, containers could only have a single network interface.
This patch allows the kube infra to support multiple interfaces
on containers. docker-spotter runs on minions and connects to the
docker daemon, receives events and executes commands on the CLI.
In our use case, docker-spotter issues a pipework command to
connect/disconnect eth1 of containers (with a name nova-network)
to the container external neutron network:

Connection pattern:
container_eth1<>kbr-ex<>kube_eth1<>neutron_ext_net