User configurable localtunnel server URL

Adds a new command line option `--tunnel-host` with a mandatory argument which should be a URL at which a localtunnel server may be found. If the option is set, that localtunnel server is used to broker the connection to Sauce Labs and when exposing a local test with `--tunnel`.

The tunnel host can also be set in `~/.zuulrc` using the `tunnel_host` field: eg. `tunnel_host: http://my.own.localtunnel.server`.

Example:
```
$ zuul --ui qunit --local --tunnel -- test/*.js 
open the following url in a browser:
https://bhcaltyqvu.localtunnel.me/__zuul

$ zuul --ui qunit --local --tunnel --tunnel-host http://privatetunnel.com -- test/*.js 
open the following url in a browser:
https://bmxjnyuhmk.privatetunnel.com/__zuul
```