Honor public_addr in signup URL.

[russjones]

Problem

When adding a user like below, the signup address that Teleport returns has the hostname of the first proxy it finds: user_command.go#L117-L135

$ tctl users add --roles=admin rjones
Signup token has been created and is valid for 3600 seconds. Share this URL with the user:
https://proxy1.example.com:3080/web/newuser/00000000000000000000000000000000

NOTE: make sure 'proxy1.example.com' is accessible!
When the user 'rjones' activates their account, they will be assigned roles [admin]

This is a problem for HA situation as the proxy host my not be directly accessible (it may be behind a LB).

Proposed Solution

If public_addr is set, honor it here as the public address of the proxy.

[kontsevoy]

Another proposed solution: maybe we should stop being too smart with our suggestions and simply print URL with <proxyhost> and say that "make sure points at Teleport proxy which users can access"