Create the directory used for rbd-clients sockets
Create the directory used for rbd-clients sockets

* `/var/run/ceph/rbd-clients` is not created automatically
* because it is missing, ceph-rgw complains about missing client
  socket on start up; it is because the containing directory is
  not there
* so we just add it to the list of directory pre-requisite

mcsage/ceph-ansible,jsaintrocc/ceph-ansible,rootfs/ceph-ansible,fgal/ceph-ansible,dang/ceph-ansible,albertomurillo/ceph-ansible,rootfs/ceph-ansible,mcsage/ceph-ansible,verdurin/ceph-ansible-1,albertomurillo/ceph-ansible,jsaintrocc/ceph-ansible,jtaleric/ceph-ansible,WingkaiHo/ceph-ansible,fgal/ceph-ansible,bengland2/ceph-ansible,travmi/ceph-ansible,WingkaiHo/ceph-ansible,WingkaiHo/ceph-ansible,ceph/ceph-ansible,albertomurillo/ceph-ansible,jfcoz/ceph-ansible,jtaleric/ceph-ansible,ceph/ceph-ansible,bengland2/ceph-ansible,font/ceph-ansible,guits/ceph-ansible,dang/ceph-ansible,jfcoz/ceph-ansible,font/ceph-ansible,travmi/ceph-ansible,verdurin/ceph-ansible-1,guits/ceph-ansible