lsb module will fallback when lsb is not found

All checks for lsb will be done in the `lsb` module and will issue logging warnings when the lsb package is not found in the remote host.

In the example output below, `node2` is a CentOS box that doesn't have lsb installed by default. Before this would not work at all and force the user to install lsb.

    $ ceph-deploy purge node2
    [ceph_deploy.install][DEBUG ] Purging from cluster ceph hosts node2
    [ceph_deploy.install][DEBUG ] Detecting platform for host node2 ...
    [ceph_deploy.lsb][WARNIN] lsb_release was not found - inferring OS details
    [ceph_deploy.install][DEBUG ] Distro CentOS codename Final
    [ceph_deploy.install][DEBUG ] Purging host node2 ...