vdsm.spec: Add BuildRequires to python-argparse

EL6 build is failing because configure.ac says argparse is fatal error
in case we don't have it in build system. This patch includes python-argparse as
BuildRequires. On the other hand, >= F19 contains python-2.7 which has
python-argparse integrated into the stdlib and don't generate such error.

Change-Id: Ie3dc596f13e934760906f43a0f16ca6c06086878
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23510
Reviewed-by: Dan Kenigsberg <danken@redhat.com>