Adds docs for the KOLLA_UPGRADE_CHECK variable, introduced here: https://review.opendev.org/c/openstack/kolla/+/878744 This feature currently hard-codes the Nova upgrade check, but should be extended in the future to support the other services that have upgrade checks. (Placement, for example: https://docs.openstack.org/placement/latest/cli/placement-status.html) The docs reflect this intention. Change-Id: I0e9d482b1b26d3e4deddbf6d4a6c83bd8e021269