Add the optional k8s-version flag to cobra

[gadelkareem]

fixes #162

Although this enables the k8s-version flag. It is not currently possible to switch k8s version because of the related config.

[mavimo]

IMHO should be a nice starting point, I think some more changes are needed:

• Update README to allow to specify what value are accepted, until now I think the cluster should be accept 1.9.X, 1.10.X and 1.11.X since config format is unchanged in this versions (as far I remember), but we should do some more checks
• E2E test should be added to allow to deploy a cluster for each supported version (maybe a small cluster for each version should be fine)
• Config should store the cluster version and it should be restored on load in order to apply consistent behaviour when needed

IMHO this should be a breaking change, so we should dump version and maybe we can consider to implement it in 0.4 where @xetys is considering to allow to have support to the K8s 1.12 (see #208), but I'm not sure we will support multiple cluster version in the application at the first iteration.

PS: @gadelkareem thx for ALL your contribution to this project :)

[gadelkareem]

@mavimo and thanks for great project ;)

I actually did test it but unfortunately all versions failed. That's why it is a good idea to leave it out of the docs for now and merge it for new upcoming versions.

Seems like 1.10 is working! Let me do some more tests and come back to you on this.

[gadelkareem]

So only v1.10* worked for me and seems to be as stable as 1.9 so far. So it is kinda good news still!

I updated the docs to add the new flag and add it to the cluster config