[CI] Stop testing lower-constraints
[CI] Stop testing lower-constraints

This patch disables the lower-constraints job as it started
failing and we agreed during the PTG to drop it anyway. [1]

[1] http://lists.openstack.org/pipermail/openstack-discuss/2020-October/018445.html

Change-Id: Icfedd16ef9aed84e9f9be939e832c35e0d735efe

openstack/kolla,stackforge/kolla,openstack/kolla,stackforge/kolla,stackforge/kolla