tool: _enable_bond_dev: reopen bonding_masters per bond

Writing multiple +bondnames into /sys/class/net/bonding_masters is not
enough to add new bonding devices. One has to reopen that file for each
added bond.

Change-Id: I4693a3d2cc3beba0b5961d16fb1ef25170f99f9e
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/12409
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Tested-by: gena cher <genadic@gmail.com>