vdsm.spec: python-ordereddict only for rhel < 7

rhel7 contains python 2.7 which already includes ordereddict module,
no need extra python package.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=891542
Change-Id: I784d82a7fb5a1c6a13f015747f077020b91c19cb
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/10649
Reviewed-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>