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
1 file changed