doc: rename manpage from `zuul` to `zuul-server`

On Debian, all binaries should have an associated manpage. While
building Zuul package we got the following lintian error:

  W: zuul: binary-without-manpage usr/bin/zuul-server

This is easily fixed by making sphinx to name the man page zuul-server.

Change-Id: Ie9fa681654a8e7e8785c1dcfa5ec21ba27ae3049
Reviewed-on: https://review.openstack.org/29287
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Paul Belanger <paul.belanger@polybeacon.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins