Network - added checks to avoid invalid configurations

- VLAN cannot be added to bond interface (bnc#790219#c22)
- check VLAN ID (integer in 0..4094 range) (bnc#790219#c28)
- check for unique VLAN ID (bnc#790219#c19)
- bond cannot be empty, at least one slave has to be present (bnc#790219#c37)