Add support for setting the domain also when a specific helper is used

Before, the domain was automatically added assuming that the
NextcloudTestServerContext had no parameters defined in the Behat
configuration. However, in order to use a helper for Apache it would
need to be specified in the configuration with something like:

- NextcloudTestServerContext:
    nextcloudTestServerHelper: NextcloudTestServerLocalApacheHelper

The substitution now works both when a helper is specified and when it
is not; note, however, that providing custom parameters to the helper is
not supported, although they are not needed anyway so it is not really a
problem.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>