Drop lower-constraints job

Lower-constraints is not a requirement of the OpenStack Python PTI
[0] and there currently is a discussion on the mailing list [1]
about dropping the test, with the oslo team already having done
so [2].

The new dependency resolver in pip fails due to incompatible
dependency versions in our lower-constraints file, meaning that
we were never providing any real guarantees with it.

To unblock the CI, I am disabling lower-constraints job for now,
with the option to reenable it in case we fix the constraints,
and based on the outcome of the mailing list discussions and
consensus.

This also marks the k2k federation job as non-voting because Xenial is
broken with Shibboleth and OpenSUSE python dependencies are currently
broken.

[0]. https://governance.openstack.org/tc/reference/pti/python.html
[1]. http://lists.openstack.org/pipermail/openstack-discuss/2021-January/019672.html
[2]. http://lists.openstack.org/pipermail/openstack-discuss/2021-January/019659.html

Change-Id: I47e747058891596e7717848a0b0bc10f0a235b2f
(cherry picked from commit d6610594d1)
(cherry picked from commit 42faeb277b)

.zuul.yaml

@@ -178,7 +178,6 @@
 - project:
     templates:
       - openstack-cover-jobs
-      - openstack-lower-constraints-jobs
       - openstack-python3-ussuri-jobs
       - publish-openstack-docs-pti
       - periodic-stable-jobs
@@ -199,6 +198,7 @@
             voting: false
             irrelevant-files: *irrelevant-files
         - keystone-dsvm-py3-functional-federation-opensuse15-k2k:
+            voting: false
             irrelevant-files: *irrelevant-files
         - keystoneclient-devstack-functional:
             voting: false
@@ -226,6 +226,7 @@
         - keystone-dsvm-py3-functional:
             irrelevant-files: *irrelevant-files
         - keystone-dsvm-py3-functional-federation-opensuse15-k2k:
+            voting: false
             irrelevant-files: *irrelevant-files
         - tempest-full-py3:
             irrelevant-files: *tempest-irrelevant-files