tests: helper to run unit / function tests in docker

For instance

   test/docker-test.sh --os-type ubuntu --os-version 14.04 \
        test/ceph-disk.sh

runs test/ceph-disk.sh in a ubuntu 14.04 docker container. Once the
container is populated and ceph compiled, running a test script roughly
requires entering the container and running make TESTS=tests/foo.sh check

* docker build ceph-ubuntu-14.04 using ubuntu.dockerfile as a Dockerfile
* it will run apt-get install ceph compilation / run dependencies
* git clone the-local-clone ceph-ubuntu-14.04
* docker run ceph-ubuntu-14.04 make -j4 in the ceph-ubuntu-14.04 clone
* docker run test/ceph-disk.sh

test/docker-test.sh is the command line interface for
test/docker-test-helper.sh which can be invoked from shell scripts.
test/ubuntu.dockerfile and test/ubuntu.dockerfile are regular
Dockerfiles which allow substitution of environment variables.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
ceph-disk: test prepare / activate on a device

This indirectly tests that partprobe is called after zap because it
would fail to map the partitions to /dev/disk/by-partuuid otherwise.

It also indirectly test the implementation of init=none when using a
block device because the test would fail to put an object into the rbd
pool using the device otherwise.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
autotools: add --enable-docker

Docker based tests should be explicit instead of auto-detected. It is
good that they do not run if docker is not available. It would be bad if
they run when the developer does not expect them to create docker
containers.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>