Create admin kubeconfig using SA token

[jakolehm]

What would you like to be added:

Admin kubeconfig should use SA token instead of client cert auth.

Why is this needed:

SA token does not expire and it can be easily revoked via kube api if needed. Cert auth does expire and revoke is not possible.

[kke]

How about generating a short lived bootstrap token for each run?

[jakolehm]

How about generating a short lived bootstrap token for each run?

With admin kubeconfig I mean the config that pharos kubeconfig fetches (which is stored in master at ~/.kube/config).