Adapt Selenium to work remotely as well as local.

Adapt Selenium to work remotely as well as local.

- Parallelel tests are not working with remote Selenium. For now
  let's detect the environment and disable parallel tests when
  remote.
- When using remote Selenium, the browser doesn't reset properly
  when changing preferences. As a result these are not applied.
  As a workaround, on remote we'll run a Selenium instance per
  different set of settings we need. This is not scalable, but
  for now it should be ok as there are only 3 sets.
- Tell WebMock not to interfere with requests between containers.