base: remove 'arg' so travis is happy
base: remove 'arg' so travis is happy

Travis is using Docker 1.8.x and thus does not support 'ARG' in the
Dockerfile. So instead re-using the old fashion way.

[ci skip]

Signed-off-by: Sébastien Han <62d754cc350e84d3b1c32ae79f976f5348e74a40@redhat.com>

ceph/ceph-docker,AcalephStorage/ceph-docker,JrCs/ceph-docker,AcalephStorage/ceph-docker,ceph/ceph-docker,cdxvirt/ceph-docker,AcalephStorage/ceph-docker,osixia/ceph-docker,JrCs/ceph-docker,mkkie/ceph-docker,rootfs/ceph-docker,osixia/ceph-docker,rootfs/ceph-docker,osixia/ceph-docker,fmeppo/ceph-docker,fmeppo/ceph-docker,JrCs/ceph-docker,fmeppo/ceph-docker,cdxvirt/ceph-docker,mkkie/ceph-docker