Expand TELEPORT_SITE environment to tsh login, fixes #2668 (#2675) (#2942)
Expand TELEPORT_SITE environment to tsh login, fixes #2668 (#2675) (#2942)

The following now works:

```
TELEPORT_SITE=two.example.com tsh login
```
gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport,gravitational/teleport