Allow container scheduling across availability zones

Container bays currently do not expose any information regarding the nodes availability zones (AZs).

A common use case is to have multiple instances of a container running on nodes in different zones.

Taking docker swarm as an example, the goal would be to use affinity filters based on labels to achieve this goal, something like:

docker run -it -d -p 80:80 --label nova.availability-zone=my-zone-a nginx
https://docs.docker.com/swarm/scheduler/filter/#use-an-affinity-filter

For this to be possible we need to add labels to the docker daemon configs, exposing availability zone information (taken from the nova instance metadata):
https://docs.docker.com/engine/userguide/labels-custom-metadata/#daemon-labels

There seem to be two changes required in magnum:

1. Add the availability zone label to the docker daemon configuration on the nodes

2. Get magnum (heat) to launch nodes across availability zones (from a predefined list, probably on the baymodel)

NOTE: As discussed at the team meeting, this blueprint requires a spec to start.