trivial: Add a note about 'cells_api'
trivial: Add a note about 'cells_api'

I was confused about what was cells v1 and what was cells v2. That got
clarified in a review [1]. Add this clarification to the code itself.

[1] https://review.openstack.org/#/c/391941/22/nova/compute/cells_api.py

Change-Id: I6db254fdaa60923732fe01ae53a98e5d7f126247

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