ceph-disk : fixes and improvements for the --data-dir code path and associated tests

This patch series is meant to make it possible to :

* use ceph-disk from sources to run an OSD for test purposes
* allow puppet-ceph to create OSDs using data directories instead of devices using ceph-disk, both for test purposes and deployment with as little changes as possible

It adds the following features and tests:

* run ceph-osd when --mark-init=none
* create the data directory if it does not exist
* tests for the --data-dir code path 
* run ceph-osd when --mark-init=none

It also fixes a number of minor bugs.