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>