extend SauceBrowser init_conf with capabilities supplied in .zuul.yml

Currently, it is not possible to pass extra capabilities (as documented at https://docs.saucelabs.com/reference/test-configuration/#selenium-2-tests-desired-capabilities) to sauce labs. This patch allows you to add a `capabilities` key in the `.zuul.yml` that is used to extend the browser config.

example config

```
ui: mocha-bdd
browsers:
  - name: chrome
    version: latest
capabilities:
  public: private
```