Use the HTTP host value specified by the user when building
notification events. This value has now a higher priority when building
the "source" part of the event notifications.
This is required when running the registry behind a web server like
Apache. Without this code all the notifications would have the source
set to "registry.example.com:5000" or "127.0.0.1:5000" instead of the
FQDN exposed by Apache (i.e. "registry.example.com").
Signed-off-by: Flavio Castelli <firstname.lastname@example.org>