This allows us to force another plugin as the default vhost implementation, or disable it altogether in favor of some higher-level proxy implementation, such as one that integrates with service discovery.
There are cases where we don't want the check for keyfile, this will add a
config option, that allows you to skip this particular check.
Fixes#1851
Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>
