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
diff --git a/doc/source/conf.py b/doc/source/conf.py
index 2e05cc2..6943bd4 100644
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -216,7 +216,7 @@
 # One entry per manual page. List of tuples
 # (source start file, name, description, authors, manual section).
 man_pages = [
-    ('index', 'zuul', u'Zuul Documentation',
+    ('index', 'zuul-server', u'Zuul Documentation',
      [u'OpenStack'], 1)
 ]