doc: add cells v2 FAQ on refreshing global cells cache
doc: add cells v2 FAQ on refreshing global cells cache

There is a cells cache in the API and in the request context,
which is used in the controller services (api, conductor, and
scheduler), so this adds a FAQ item about needing to restart
those services if you update the db connection or transport url
in a given cell which is already cached.

Change-Id: I56daf87dad9e86f747e945f0842344ddba42b91e

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