Only assemble {{ cluster }}.conf and osd.conf

Only assemble {{ cluster }}.conf and osd.conf

Ansible's assemble module by default will put all files in the src
directory together into dest. We only want to put {{ cluster }}.conf
and osd.conf together, not anything that might have found its way into
/etc/ceph/ceph.d (e.g. files left by the sysadmin taking backups
before an ansible run). So specify a regexp that matches only those
two files.

Signed-off-by: Matthew Vernon <mv3@sanger.ac.uk>