Names of labels should be configurable

[tomislater]

Now, we have this:

# TODO: this should be configurable
NODE_LABEL_SPOT = "aws.amazon.com/spot"
NODE_LABEL_ROLE = "kubernetes.io/role"
# the following labels are used by both AWS and GKE
NODE_LABEL_REGION = "failure-domain.beta.kubernetes.io/region"
NODE_LABEL_INSTANCE_TYPE = "beta.kubernetes.io/instance-type"

but, some people use different labels.

[tomislater]

I am going to fix it :P