Regression test portability: rpmbuild

rpmbuild is required, check for its presence.

BUG: 1129939
Change-Id: Iae8534bf897a4605620277688a180d6542c1c80b
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8810
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Tested-by: Harshavardhana <harsha@harshavardhana.net>