Add pause/unpause needed by OpenStack Nova Docker driver

Nova Docker driver has a custom http client which we are trying
to replace with docker-py and during the initial review, pause
and unpause seem to be missing from docker-py.

https://github.com/stackforge/nova-docker/blob/master/novadocker/virt/docker/client.py#L177

The Docker Remote API documentation is here:
https://docs.docker.com/reference/api/docker_remote_api_v1.14/#pause-a-container
https://docs.docker.com/reference/api/docker_remote_api_v1.14/#unpause-a-container