Fix is_package_installed() check with dpkg

is_package_installed() incorrectly returned '0' for packages that
had 'un' status in the dpkg database.

Change-Id: I81b77486c2ed7717ed81cb2c2572fe6c4b394ffc
Add support for Docker as Nova hypervisor

* Add basic support for hypervisor plugins in lib/nova_plugins
* Add lib/nova_plugins/hypervisor-docker to use Docker as a Nova
  hypervisor.
* Add tools/install_docker.sh to install the Docker daemon and
  registry container, download base image and import
* Configure Nova to use docker plugin
* Add docker exercise and skip unsupported ones

Nova blueprint: new-hypervisor-docker

Change-Id: I9e7065b562dce2ce853def583ab1165886612227
Merge branch '38127-testonly' into dt-test
Merge branch 'docker' into dt-test