zuul-server learned --version

I have so many Zuul installation that I somehow loose track of their
version. I am forced to look at the zuul-server wrapper script to figure
out the current version when I could just ask the might zuul-server to
give me its version straight from the command line.

Example:

 $ zuul-server --version
 Zuul version: 1.1.0.45.g70c7158

Since we might not have a valid zuul.conf passed, the version display
must happen before  server.read_config() or we end up with a stracktrace
on a fresh installation.

Change-Id: I284de8da5af6678c7aaa1b17c950390e135f6e89
Reviewed-on: https://review.openstack.org/25007
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Elizabeth Krumbach <lyz@princessleia.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins