Serialize default_gateway/other_nets for 6.0 only

Both default_gateway and other_nets fields have been introduced in Fuel 6.0,
but accidently were serialized for old envs too. This led to puppet
errors when we tried to re-deploy node in one of the old envs.

Change-Id: I7e4ba8f3a6a0ba822c47b9c14fe4d02b3523d658
Closes-Bug: #1403560