iscsi-gw: Create a rbd pool if it doesn't exist
iscsi-gw: Create a rbd pool if it doesn't exist

iscsi-gw needs a 'rbd' pool to configure iscsi target.
Note: I could have used the facts already set in `ceph-mon` but I voluntarily
didn't do it to not create a dependancy between these two roles.

Signed-off-by: Guillaume Abrioux <30a9df5a0293a6108675377fa1673f7aa713f915@redhat.com>

travmi/ceph-ansible,fgal/ceph-ansible,fgal/ceph-ansible,ceph/ceph-ansible,font/ceph-ansible,font/ceph-ansible,jtaleric/ceph-ansible,travmi/ceph-ansible,ceph/ceph-ansible,jtaleric/ceph-ansible