functional tests for production servers

With our setups getting richer and richer, it makes sense to run functional tests that go beyond the manual smoke tests (push/pull with both protocols) that I've been conducting for upgrades of foss.h.n and heptapod.host.

This introduces an operation mode (--heptapod-prod-server) in which the functional tests don't use the full administrative rights, contenting themselves with an user that owns some group, and a subcategory of tests to run on production servers (direct flagging with @suitable.prod_server).

Had a successful run on foss.heptapod.net, using the heptapod/testing-projects/automated subgroup.