network: Don't remove bonding module on cleanup

Previously the bonding module got unloaded to remove all bonds on the
system, but this conflicted with other parts of Node, namely vdsm.
Therefor now the bonds are removed by bringing down each device on it's
own.

Change-Id: Ia2eada10beeb981ecf6b4dbe1b51d2c38f8daea7
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=995155
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>