Add some notes on DISTRO setting
Add some notes on DISTRO setting

It was pointed out in I42599c1bd851d2b61a56a3960f42ad051aab1727 that
after I46a2c36d95327087085df07cb797eb91249a893c we now die if we can't
determine DISTRO, rather than take a guess.

After some consideration, I don't consider this a regression.  This
default matching was basically only working for LinuxMint, because we
already have some matching there for setting os_PACKAGE that makes us
know the platform is Ubuntu-ish.

However, I would certainly agree it is not particularly clear as to
why.  This just adds some comments explaining why we can not just add
a default DISTRO fall-through and directs implementors on how to add
support for their platform.

Change-Id: I4b07259be1427d86a7154906646073d08dd07294

openstack-dev/devstack,bswartz/devstack,bigswitch/devstack,Mahito/devstack,samgoon/devstack,nawawi/openstack,group-policy/devstack,openstack-dev/devstack,richliu/devstack,bswartz/devstack,richliu/devstack,nawawi/openstack,group-policy/devstack,bigswitch/devstack,samgoon/devstack,noironetworks/devstack,Mahito/devstack,noironetworks/devstack,dtroyer/devstack,dtroyer/devstack