Pin pip to <8 for openstackci-beaker jobs

These jobs install pip packages system-wide but some dependencies
conflict with some things that we still have installed in packages.
Pin pip to < version 8 until we can fix that.

Change-Id: Ic3ae09df09be0c06cb0735a06d9fb082e78d11d1

jenkins/jobs/infra.yaml

@@ -144,6 +144,7 @@
       - print-template-name:
           template-name: "{template-name}"
       - link-logs
+      - pin-pip
       - zuul-clone:
           project: openstack-infra/puppet-openstackci
       - puppet-prepare-node

jenkins/jobs/macros.yaml

@@ -310,6 +310,11 @@
             erb -x -T '-' $f | ruby -c
           done
 
+- builder:
+    name: pin-pip
+    builders:
+      - shell: sudo pip install "pip<8"
+
 # note that this job uses an in-repo script for execution
 # in order to cater for differing requirements per branch
 - builder: