Add introduce doc how to generate magnum.conf.sample
Add introduce doc how to generate magnum.conf.sample

Now magnum.conf.sample need to generate by tox -egenconfig
It is convenient to have a help doc in the directory etc/magnum/

Change-Id: Ia18db548e8840bbccc915b58b7a2301043533469

jay-lau/magnum,openstack/magnum,ArchiFleKs/magnum,openstack/magnum,ArchiFleKs/magnum