engine: Block labeling a bond without slaves

SetupNetworks doesn't support a bond without at least
2 slaves, therefore such bonds shouldn't be labeled.

Bug-Url: http://bugzilla.redhat.com/1054187
Change-Id: Ifeb8854797d397aaaf23001ff481b39916289445
Signed-off-by: Moti Asayag <masayag@redhat.com>