cthulhu: absorb the salt from rest-api to get logs when the www user lacks privs to get them

Signed-off-by: Gregory Meno <gmeno@redhat.com>
Merge pull request #159 from ceph/wip-fix-logs

cthulhu: add log service function to avoid privilege probs calling salt directly from REST

Reviewed-by: Dan Mick <dan.mick@inktank.com>