Initial support for running the test harness in a Docker container

Using the "contained_harness" ksmeta variable, one can run the tests in
a Docker container instead of the real test system.

Bug: 1131388
Change-Id: Ib1bd525e07b32c2c788ef68021cfb5576d0dcf63
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: Iab6321d379014cad252bd58f57d09dac9433108f
Bug: 1148673
Support running "beah" test harness in a Docker container

Commit bea3655 added support for running the test harness in a docker
container. However, the "beah" test harness needed some additional
work. This patch does that.

Non-systemd docker images are not supported yet.

Bug: 1131388
Change-Id: If49372fa879c91b997ff79d84a8458c5241fa136
log unhandled exceptions in beaker-provision main loop

Bug: 1142533
Change-Id: Ib0b8c1401f7528514481678d56f4c86bf0e44cc5
Document running test harness in a container + atomic provisioning

Bug: 1148673
Bug: 1131388

Change-Id: Ibebd61ccc040a2240257a92a35a5347a8062ffa2
release notes for 0.18.3

Change-Id: I20845771c3be6c4f7035380005764126a1bf993d
Automatic commit of release 0.18.3