Experimental support for provisioning rpm-ostree based distros

This patch adds support for provisioning "rpm-ostree" based Project
Atomic distributions. The feature variable "has_rpmostree" should be used to
identify such a distribution. On such distros, the test harness will be
run in a Docker container rather than on the host system itself.

Change-Id: Ia4eb796b603ed2f1e9a4b60b773a0e82af6522e8
Bug: 1148673