server: add property to specify the server base URL

This is not used per se by the container, it is
for informational purposes only. If you have several servers,
you probably want to put the VIP address here.

The first usecase will be for the Swagger integration.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
Merge branch 'master' of github.com:killbill/killbill-platform