Make providing container registry credentials easy via the helm chart

[consideRatio]

BinderHub spawns Build pods that communicates with the docker-api setup on the k8s node's, and they need to pass credentials to push to an external container registry. So, for this to work, BinderHub starts these pods up to mount a k8s Secret with a docker client config file including credentials.

1. A person installing the chart should be able to provide server / username / password to a container registry and have BinderHub spawned build pods able to push to it.
   I've not got the option to work out, such as kubectl create secret docker-registry <name> --docker-server=... and then reference that k8s Secret.
2. We should support re-using previously provided credentials
   This makes someone do initial setup with helm install --set ... and then not need to retain the sensitive credentials in a git repo etc. We do this for a lot of z2jh credentials already for example.
3. We should support having these credentials cleaned up when the helm release is deleted
   This comes out of the box by managing the k8s Secret we create via the Helm chart

[damianavila]

@consideRatio, this is fixed by #36, right?

[consideRatio]

@damianavila yes thanks thats correct!