The following changes have been introduced
to tsh login behavior:
1. tsh login now accepts cluster name
as an optional positional argument:
$ tsh login clustername
2. If tsh login is called without arguments
and the current credentials are valid,
tsh login now prints status, previous behavior
always forced login:
$ tsh login
... print status if logged in...
2. If tsh login is called with the proxy
equal to current, tsh login selects cluster,
otherwise it will re-login to another proxy:
$ tsh login one
... selected cluster one
$ tsh login two
... selected cluster two
$ tsh login --proxy=example.com three
... selected cluster three because
proxy is the same
$ tsh login --proxy=acme.example.com four
...will switch to proxy acme.example.com
and cluster four
