add security hardening defaults
Add some security hardening defaults as recommended in Google's hardening guide. Some guidelines are not followed at this time for technical reasons as discussed in the README. We may be able to use them in the future if we find a new way for the terraform k8s provider to talk to the cluster.
Add a note to the README on how to do minimal downtime node pool upgrades, add a CHANGELOG and bump the major version number as the security hardening defaults will almost certainly require some manual upgrade of clusters.
Closes #21 (closed)
Closes #20 (closed)