Tests: Update from cockpit-deps.

So that packages in cockpit-deps can update packages in the base
tarball.
Tests: Work around bug in shutdown sequence.
Tests: Refactor test machine setup.

To prepare for OS specific tweaks.
Tests: Support Fedora-20.
Tests: Use a fixed host key for the test machines.

The alternative is to use UserKnownHostsFile=/dev/null and somehow get
rid of the "Warning: Permanently added ..." messages.