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

This reverts commit 91e8103794349ebdd578b77987a089456a59c52b

Reverting as regression tests are failing after this merge.

Change-Id: I9dcd430b2f9a431c3c01deab1a205aed297216a3
Reviewed-on: http://review.gluster.org/5842
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>