Automatic merge from submit-queue (batch tested with PRs 43000, 44500, 44457, 44553, 44267)

Add Kubernetes 1.6 support to Juju charms

**What this PR does / why we need it**:

This adds Kubernetes 1.6 support to Juju charms.

This includes some large architectural changes in order to support multiple versions of Kubernetes with a single release of the charms. There are a few bug fixes in here as well, for issues that we discovered during testing.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

**Special notes for your reviewer**:

Thanks to @marcoceppi, @ktsakalozos, @jacekn, @mbruzek, @tvansteenburgh for their work in this feature branch as well!

**Release note**:

```release-note
Add Kubernetes 1.6 support to Juju charms
Add metric collection to charms for autoscaling
Update kubernetes-e2e charm to fail when test suite fails
Update Juju charms to use snaps
Add registry action to the kubernetes-worker charm
Add support for kube-proxy cluster-cidr option to kubernetes-worker charm
Fix kubernetes-master charm starting services before TLS certs are saved
Fix kubernetes-worker charm failures in LXD
Fix stop hook failure on kubernetes-worker charm
Fix handling of juju kubernetes-worker.restart-needed state
Fix nagios checks in charms
```