Add 'delete_host' command in 'nova-manage cell_v2'.
Add an optional 'force' option in 'nova-manage cell_v2 delete_cell'.
If specifying the 'force' option, a cell can be deleted
even if the cell has hosts.
Change-Id: I8cd7826c2c03687c6b85731519778f09d542b236
Closes-Bug: #1721179