make the promisc property compliant with old pyflakes

el6 pyflakes is older than the addition of setters and deleters for
python class properties. Thus, it wrongly detects the syntax:

    @x.setter
    def x(self, value)

As a redefinition of x. Thankfully, it is possible to use
the property builtin to achieve the same result without being
flagged as re-defining.

Change-Id: Ib44e44f12e15f6613cb9c841dc11938ab0ce9755
Bug-Url: https://bugzilla.redhat.com/1134851
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33071
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>