Override current-context of garden cluster kubeconfig

[petersutter]

What this PR does / why we need it:
This PR allows to override the current context of the garden cluster kubeconfig.

Example:

gardens:
- identity: landscape-dev # Unique identity of the garden cluster. See cluster-identity ConfigMap in kube-system namespace of the garden cluster
  kubeconfig: ~/path/to/garden-cluster/kubeconfig.yaml
  context: different-context # Overrides the current-context of the garden cluster kubeconfig  

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

You can now override the current context of the `garden` cluster `kubeconfig` by setting the `gardens[].context` property in your `gardenlogin` config file.

[holgerkoser]

\lgtm

[grolu]

/lgtm