Fix ping endpoint when proxy has multiple public addrs

[r0mant]

Fixes a small regression introduced by app access in Teleport 5.0 where if the proxy has multiple public addresses configured, tsh login using any other than the first one would fail.

Suppose my proxy has the following config:

  public_addr:
  - root-1.gravitational.io:3080
  - root-2.gravitational.io:3080

Without this fix:

➜  ~ tsh login --proxy=root-2.gravitational.io:3080 --auth=okta
ERROR: invalid character '<' looking for beginning of value

With this fix:

➜  ~ tsh login --proxy=root-2.gravitational.io:3080 --auth=okta
If browser window does not open automatically, open it by clicking on the link:
 http://127.0.0.1:52067/13d60145-7a9e-45c7-9d98-6ac01efc2655
> Profile URL:        https://root.gravitational.io:3080
  Logged in as:       <user>
  Cluster:            root
  Roles:              admin
  Logins:             root, ubuntu
  Kubernetes:         disabled
  Valid until:        2021-06-22 02:47:08 +0300 MSK [valid for 12h0m0s]
  Extensions:         permit-agent-forwarding, permit-port-forwarding, permit-pty

[smallinsky]

Code LGMT

[russjones]

Bot.