nova-manage docs: cell_v2 create_cell
nova-manage docs: cell_v2 create_cell

Document the `nova-manage cell_v2 create_cell` command.

This is part of a series to document the nova-manage cells
commands.

Change-Id: Ic9ddef319aaf8a377d733633cc279a0fc1558cb2

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