tests/basic/rpm.t: run mock builds in parallel

save about six minutes per regression by building in parallel

Change-Id: Iaf6fa545e20b0322b10e9e4ab7cc3d4254a0ece5
BUG: 1006269
Signed-off-by: Kaleb S. KEITHLEY <kkeithle@redhat.com>
Reviewed-on: http://review.gluster.org/5845
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>