Automated cherry pick of #8373: Prefix git tags with `v`

Automated cherry pick of #8338: Don't load nonexistent calico-client cert when CNI is Cilium

Go modules require the v prefix [0], and k/k also tags with the v prefix [1]

We have some inconsistent tags already, for 1.11.0 we have tags of both `1.11.0` and `v1.11.0` which is the most recent tag with the prefix.
This is also why 1.11.0 is the default version imported by `go get`:

```
go get -v k8s.io/kops
go: downloading k8s.io/kops v1.11.0
```

and the latest version in `go list`:

```
go list -m -versions k8s.io/kops
k8s.io/kops v1.4.0-alpha.1 v1.4.0 v1.4.1 v1.4.2 v1.4.3 v1.4.4 v1.10.0 v1.11.0
```

I'm proposing we switch to only tagging with the v prefix. I'm only updating the actual git tag and not the entire version string used throughout kops due to its larger impact:

* Output by `kops version`
* Public URLs for kops assets
* Protokube tag

I'm hoping this is the least invasive way we can make this change.
If we think advanced notice is required, we could tag with both formats for a number of releases before tagging only with the v prefix.

[0] https://github.com/golang/go/wiki/Modules#modules
[1] https://github.com/kubernetes/kubernetes/releases/tag/v1.17.0

Automated cherry pick of #8333: CoreDNS default image bump to 1.6.6

Also updates the default corefile config to make use of the new lameduck functionality for healthcheck

Automated cherry pick of #8248: Fix issues with older versions of k8s for basic clusters

Automated cherry pick of #8317: Don't output empty sections in the manifests

Automated cherry pick of #8378: Backport the k8s 1.9 required action release note

Cherry pick #7874 onto 1.15

(The tests are split out for easier cherry-picking, should we choose to)

In our kube-dns manifest for 1.6 we often had an empty section,
normalization converted this to `{}` which causes `kubectl apply` to
fail.

We can simply skip empty objects when outputing.

Automated cherry pick of #7916: Fix rendering of the Node Authorizer template

Cherry pick of #8241 into 1.15: Update copyrights for 2020

Automated cherry pick of #7832: cilium: don't try to mount sys/fs/bpf if already mounted

mark weavenet-pod as system-critical

systemd v238 already includes the mount, and the unit file fails.  We
test for the existence of the mount, rather than testing systemd
versions directly.

Automated cherry pick of #8022: Bump cilium version to 1.6.4

Automated cherry pick of #8115 Openstack: Fix cluster floating ips

Automated cherry pick of #7545: Complete support for Flatcar

Signed-off-by: Salvatore Mazzarino <dev@mazzarino.cz>

Signed-off-by: Salvatore Mazzarino <dev@mazzarino.cz>

Signed-off-by: Salvatore Mazzarino <dev@mazzarino.cz>

Signed-off-by: Salvatore Mazzarino <dev@mazzarino.cz>

Cherry pick of #8062 to release-1.15

(cherry picked from commit dfee0547)

(cherry picked from commit f21df7cd)