Run ERP5 tests under usershosts wrapper with the same host mapping as zope instances.
This will allow to simplify configuration of services such as memcached and also be a more realistic environment.
This revealed problems with userhosts, at least on Debian 9, that have been fixed in https://lab.nexedi.com/nexedi/userhosts/merge_requests/2 , so at the same time we update userhosts to get these fixes.