Provide instruction to generate kolla-build.conf

Although not strictly mandatory, it's less confusing to have
a kolla-build.conf file in /etc/kolla that users can modify it they
want to customize their builds, rather than to rely on defaults
settings and command line options.

The `tox -e genconfig` was already documented in "Image building" doc.

Change-Id: Ia44e6f28ce18601cdaba5109ea12cd50fa8372b2
Closes-Bug: #1532066

doc/quickstart.rst

@@ -92,7 +92,9 @@ Copy Kolla configuration to /etc:
 
 ::
 
-    cp -r kolla/etc/kolla /etc/
+    cd kolla
+    cp -r etc/kolla /etc/
+    tox -e genconfig
 
 Since Docker is required to build images as well as be present on all deployed
 targets, the Kolla community recommends installing the official Docker, Inc.