Write and symlink kubeconfig on change of target

[holgerkoser]

What this PR does / why we need it:
With this PR it is possible to change the target cluster of kubectl directly with the gardenctl target ... command without having to adjust the KUBECONFIG environment variable again and again. Whether the current target cluster of gardenctl and kubectl should be synchronized or not can be set using the configrations property link-kubeconfig or via environment variable GCTL_LINK_KUBECONFIG.

Behavior without symlink | linkKubeconfig: false

The target command only changes the current target cluster of gardenctl, but does not download the corresponding kubeconfig file. This happens only when the kubectl-env command is executed. If the user evals the generated script the KUBECONFIG environment variable points the downloaded kubeconfig file for the targeted cluster.

The generated script looks something like this:

export KUBECONFIG=$TMPDIR/garden/<gctl-session-id>/kubeconfig.<hash>.yaml

Behavior with symlink | linkKubeconfig: true

The target command changes the current target cluster of gardenctl, downloads the corresponding kubeconfig file and creates a stable symlink to the current kubeconfig file in the session directory. If the target is changed the symlink points to the new target but the KUBECONFIG environment variable does not need to be changed. The kubectl-env command now does nothing except create the script to set the KUBECONFIG environment variable. This works even if nothing is targeted and could be done in the shell profile.

The generated script only needs to be evaluated once per shell session and looks something like this:

export KUBECONFIG=$TMPDIR/garden/<gctl-session-id>/kubeconfig.yaml

Test

Test are not yet changed !!!

Which issue(s) this PR fixes:
Fixes #81

Special notes for your reviewer:

Release note:

[RaphaelVogel]

nice, waiting to get this feature 👍

[petersutter]

/lgtm