Fix kubeconfig merge with existing config

[alankan-finocomp]

Changes

Since the existing config file was not in scope, the commands did not perform anything.

To fix it, I include the existing kubeconfig path in KUBECONFIG variable such that the YAML contents are merged when it prints out via kubectl config view --flatten

Also, replacing keyword "default" with ansible variable kube_config can also help replace the current-context in the kubeconfig file.

Linked Issues

#350

[dereknola]

This function is not intended to merge with the default kubeconfig. That's why there is an explicit check
This seems to be an error with the if statement, it should be reversed.
https://github.com/k3s-io/k3s-ansible/blob/master/roles/k3s_server/tasks/main.yml#L160

The intention is that if supply a dedicated kubeconfig location for the k3s-ansible controlled clusters, we don't do any merging or overriding.

[dereknola]

In order for this to not affect user defined kubeconfigs, we need to check and see if the kubeconfig is still equal to our variable default.

[dereknola]

We also need to override the default kubeconfig at the end

[joaogbcravo]

There is also the block which can cause a full overwrite.

[rpressiani]

This was a good PR, any chance we can accept the changes and merge it? @dereknola @alankan-finocomp

[dereknola]

I can take it over and fix it up, likely sometime next week when I have free time.