doc: add FAQ entry for cells v1 config options
doc: add FAQ entry for cells v1 config options

It's super confusing for new people to nova and cells
to see config options in a [cells] group and think they
should be changing those, like enable and cell_type.

While we have warnings in the config option help text,
let's also put a reminder in the FAQ page in the docs.

Change-Id: I5e106d9e0743d918e2115d809ac3732c5a3d7a5a

phenoxim/nova,rahulunair/nova,jianghuaw/nova,vmturbo/nova,mikalstill/nova,jianghuaw/nova,vmturbo/nova,mahak/nova,rahulunair/nova,klmitch/nova,gooddata/openstack-nova,mikalstill/nova,rahulunair/nova,klmitch/nova,klmitch/nova,vmturbo/nova,mikalstill/nova,vmturbo/nova,mahak/nova,openstack/nova,jianghuaw/nova,jianghuaw/nova,openstack/nova,openstack/nova,gooddata/openstack-nova,phenoxim/nova,mahak/nova,gooddata/openstack-nova,klmitch/nova,gooddata/openstack-nova