docs: Rewrite 'nova-manage placement' docs
docs: Rewrite 'nova-manage placement' docs

Change-Id: Ie973de1eb411db6379b544987b55199913eb62b6
Signed-off-by: Stephen Finucane <4725dc7f3632bf126d9fbdc30c347e91e8d2fe16@redhat.com>

mahak/nova,openstack/nova,openstack/nova,openstack/nova,mahak/nova,mahak/nova