spec: add BuildRequires: rpm-python

During the make check, the tests modules can import
caps.py which uses rpm python module and we should set
BuildRequires for it. This patch adds the BuildRequires.

Change-Id: Id12f47c3468e733ecaf4522c91e86c089b049e83
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=837014
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27059
Reviewed-by: Dan Kenigsberg <danken@redhat.com>