Make k3s version configurable

[negz]

Fixes #46

Previously there was a hidden --kubernetes-version flag that was ignored. This commit plumbs it through to the version of k3s that is installed. I noticed in #46 there was some talk of automatically detecting k3s versions (presumably to always install the latest version by default?). That functionality would be handy, but in the meantime I feel this PR is a step in the right direction, as it makes it possible to select a different version rather than having it hardcoded.

Now, when I don't supply a version I see the same behavior I used to (i.e. k3s v1.22.x is installed).

$ ./_output/binaries/colima-Darwin-arm64 start --runtime containerd --with-kubernetes -c 4 -m 4
INFO[0000] starting colima                              
INFO[0000] creating and starting ...                     context=vm
INFO[0021] starting ...                                  context=containerd
INFO[0026] waiting for startup to complete ...           context=containerd
INFO[0027] downloading and installing ...                context=kubernetes
INFO[0032] loading oci images ...                        context=kubernetes
> unpacking docker.io/rancher/mirrored-library-busybox:1.32.1 (sha256:6b23544141396cd3a36fa1b3ed3ccade0d30068c1802bbae64eef1e3e5ab9d54)...done
> unpacking docker.io/rancher/mirrored-library-traefik:2.5.0 (sha256:8a718325f4c589a2e44688ae87f74dab4c64648431ab9457dee9aa96aad93354)...done
> unpacking docker.io/rancher/mirrored-metrics-server:v0.5.0 (sha256:1a89693d2093a62715e24714fcc88f980c144fe2a769531c62a487cd1dcd5b3c)...ctr: failed to resolve rootfs: content digest sha256:ee11f1e38ac66c96e91e9504296e77a57733101b02a678
WARN[0034] error loading oci images: exit status 1       context=kubernetes
WARN[0034] startup may delay a bit as images will be pulled from oci registry  context=kubernetes
INFO[0034] starting ...                                  context=kubernetes
INFO[0039] updating config ...                           context=kubernetes
INFO[0040] Switched to context "colima".                 context=kubernetes
INFO[0040] done                                         

$ ./_output/binaries/colima-Darwin-arm64 ssh
colima:/Users/negz/control/abiosoft/colima$ k3s --version
k3s version v1.22.4+k3s1 (bec170bc)
go version go1.16.10

However, when I supply the --kubernetes-version flag I can pick a different K3S version:

$ ./_output/binaries/colima-Darwin-arm64 start --runtime containerd --with-kubernetes --kubernetes-version v1.23.4+k3s1 -c 4 -m 4
INFO[0000] starting colima                              
INFO[0000] creating and starting ...                     context=vm
INFO[0021] starting ...                                  context=containerd
INFO[0026] waiting for startup to complete ...           context=containerd
INFO[0026] downloading and installing ...                context=kubernetes
INFO[0037] loading oci images ...                        context=kubernetes
> unpacking docker.io/rancher/local-path-provisioner:v0.0.21 (sha256:7d89362db230fd1178ba9465bc3582b09872ee6be72037e2ad80cf812751299e)...done
> unpacking docker.io/rancher/mirrored-coredns-coredns:1.8.6 (sha256:a6d8488c231616918f517bd33321ade37f6b55e9355450cbf512d053e4df505e)...ctr: failed to resolve rootfs: content digest sha256:edaa71f2aee883484133da046954ad70fd6bf1fa42e5ae
WARN[0039] error loading oci images: exit status 1       context=kubernetes
WARN[0039] startup may delay a bit as images will be pulled from oci registry  context=kubernetes
INFO[0040] starting ...                                  context=kubernetes
INFO[0045] updating config ...                           context=kubernetes
INFO[0046] Switched to context "colima".                 context=kubernetes
INFO[0046] done                                         

$ ./_output/binaries/colima-Darwin-arm64 ssh
colima:/Users/negz/control/abiosoft/colima$ k3s --version
k3s version v1.23.4+k3s1 (43b1cb48)
go version go1.17.5

[abiosoft]

This is being implemented as part of #251.