engine: Unlabel removed bonds

If a bond is broken, but the bond itself is being
maintained without its slaves, the labels should
be cleared from the bond so other interfaces could
be labeled with those labels.

Change-Id: I054d757bef8c59b42456917d5824ce75d8696fa4
Signed-off-by: Moti Asayag <masayag@redhat.com>