Add support for a profile specific kubeconfig file.

[Joerger]

When a user logs in to a kube cluster with tsh login --kube-cluster=kube01 or tsh kube login kube02, in addition to updating their ~/.kube/config or $KUBECONFIG, a standalone kubeconfig for the given cluster will be generated in ~/.tsh/keys/${PROXY}/${USER}-kube/${TELE_CLUSTER}/kube01-kubeconfig. This kubeconfig has a single cluster (kube01) which is set as the context.

After logging in to a kube cluster, a user can then run tsh env to get and run export KUBECONFIG=~/.tsh/.../kube01-kubeconfig.

Example:

> tsh kube ls
Kube Cluster Name Selected 
----------------- -------- 
kube01            *        
kube02   

> eval $(tsh env)

> echo $KUBECONFIG
~/.tsh/keys/proxy.example.com/dev-kube/example.com/kube01-kubeconfig

> tsh kube login kube02
ERROR: profile specific kubeconfig is in use, run 'eval $(tsh env --unset)' to switch contexts to another kube cluster

#### Change windows/tabs

> tsh kube login kube02
Logged into kubernetes cluster "kube02"

> tsh kube ls
Kube Cluster Name Selected 
----------------- -------- 
kube01                     
kube02            *        

#### Change back to the first window

> tsh kube ls
Kube Cluster Name Selected 
----------------- -------- 
kube01            *        
kube02      

Closes #7202

[Joerger]

@klizhentas Please take a look when you can