Run through Docker with service principal and least-priviledge

[fgauna12]

For restrictive environments, create a service principal with read-only permissions and run the Docker container.
Probably certificate-based authentication would be the most kosher.

For example:

docker run -it -v /tmp/aks-health-check.pem:/cert/sp.pem boxboat/aks-health-check login -g <cluster_resource_group> -n <cluster_name> -sp <service_pricinpal_name> --cert-path /cert/sp.pem

$ aks-hc check

or

docker run -it -v /tmp/aks-health-check.pem:/cert/sp.pem boxboat/aks-health-check login -g <cluster_resource_group> -n <cluster_name> -sp <service_pricinpal_name> --cert $CERT

$ aks-hc check

[fgauna12]

This did not work out so well. We're going to revisit this.
See #17