nova-manage: Update deprecation timeline
nova-manage: Update deprecation timeline

It's not yet possible to remove these commands as DevStack relies on
them for cells v1. Let's reflect reality.

Change-Id: I9b830eea62dea4b8d8922bd73ae6bfbe4192837e
TrivialFix

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