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