Remove debian-stable, add focal/bullseye testing

OpenDev infra will soon no longer build Debian Stretch (oldoldstable)
images; remove testing here.

While we are here, do an update and add Ubuntu Focal testing and put a
Debian bullseye test in the gate; both are stable production OpenDev
image.

Recent issues with the wheel mirrors have revealed that we were
relying on pre-built wheels in testing.  Add a bindep.txt so that our
testing binary dependencies are listed.

Change-Id: Ifafeed6777e85efb3f71acc79a5ea9e8864a8cf6

.zuul.yaml

@@ -13,6 +13,8 @@
         - dib-nodepool-functional-openstack-centos-8-src
         - dib-nodepool-functional-openstack-ubuntu-xenial-src
         - dib-nodepool-functional-openstack-ubuntu-bionic-src
+        - dib-nodepool-functional-openstack-ubuntu-focal-src
+        - dib-nodepool-functional-openstack-debian-bullseye-src
     gate:
       jobs:
         - opendev-buildset-registry
@@ -21,11 +23,11 @@
         - dib-nodepool-functional-openstack-centos-8-src
         - dib-nodepool-functional-openstack-ubuntu-xenial-src
         - dib-nodepool-functional-openstack-ubuntu-bionic-src
+        - dib-nodepool-functional-openstack-ubuntu-focal-src
+        - dib-nodepool-functional-openstack-debian-bullseye-src
     experimental:
       jobs:
         - opendev-buildset-registry
         - nodepool-build-image-siblings
-        - dib-nodepool-functional-openstack-debian-stretch-src
-        - dib-nodepool-functional-openstack-debian-buster-src
         - dib-nodepool-functional-openstack-opensuse-15-src
         - dib-nodepool-functional-openstack-opensuse-tumbleweed-src

bindep.txt

@@ -0,0 +1,3 @@
+build-essential [platform:dpkg test]
+python3-all-dev [platform:dpkg test]
+libffi-dev [platform:dpkg test]