Distinguish between cells v1 and v2 in upgrades doc
Distinguish between cells v1 and v2 in upgrades doc

The note about rolling upgrades not being supported for cells
was referring to cells v1, so let's make that clear.

We will support rolling upgrades for cells v2 deployments, we
just haven't documented it yet (or done it in a grenade job).

Change-Id: I6a8b3d3aaa49859e8b769a296684ac96883719f8

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