devref: update the nova architecture doc
devref: update the nova architecture doc

Updated to reflect Nova's current relationship with the database.
Also adding references to work on cells.

Change-Id: I7dcf9378fdd1c1f62a51944ea64ac20f8cdfe0d6
mahak/nova,alaski/nova,zhimin711/nova,BeyondTheClouds/nova,rahulunair/nova,sebrandon1/nova,NeCTAR-RC/nova,Juniper/nova,hanlind/nova,Juniper/nova,Juniper/nova,klmitch/nova,alaski/nova,sebrandon1/nova,devendermishrajio/nova,apporc/nova,Juniper/nova,klmitch/nova,NeCTAR-RC/nova,BeyondTheClouds/nova,gooddata/openstack-nova,jianghuaw/nova,hanlind/nova,devendermishrajio/nova,rajalokan/nova,hanlind/nova,dims/nova,mikalstill/nova,phenoxim/nova,vmturbo/nova,vmturbo/nova,mikalstill/nova,cloudbase/nova,rajalokan/nova,jianghuaw/nova,CEG-FYP-OpenStack/scheduler,apporc/nova,mikalstill/nova,openstack/nova,cernops/nova,rahulunair/nova,rahulunair/nova,zhimin711/nova,cloudbase/nova,phenoxim/nova,jianghuaw/nova,openstack/nova,bigswitch/nova,mahak/nova,klmitch/nova,bigswitch/nova,klmitch/nova,vmturbo/nova,openstack/nova,vmturbo/nova,jianghuaw/nova,cyx1231st/nova,rajalokan/nova,cernops/nova,cernops/nova,cloudbase/nova,cyx1231st/nova,gooddata/openstack-nova,rajalokan/nova,gooddata/openstack-nova,sebrandon1/nova,BeyondTheClouds/nova,CEG-FYP-OpenStack/scheduler,dims/nova,gooddata/openstack-nova,mahak/nova