Horizon: Install pymemcached

... so that we can use PyMemcacheCache backend. The MemcachedCache
backend, which has been used previously, has been removed in recent
Django, and we are switching the default backend in [1].

[1] https://review.opendev.org/c/openstack/horizon/+/891828

Change-Id: Ie1da8970628e34c41721198cdada8c7bb3b26ec0

lib/horizon

@@ -169,6 +169,10 @@ function install_horizon {
     # Apache installation, because we mark it NOPRIME
     install_apache_wsgi
 
+    # Install the memcache library so that horizon can use memcached as its
+    # cache backend
+    pip_install_gr pymemcache
+
     git_clone $HORIZON_REPO $HORIZON_DIR $HORIZON_BRANCH
 }