Added selective public network serialization

Added hepler method objects.Node.should_have_public() to determine
is it required to assign Public network IPs to particular node.
It is checked on IPs assignment and deployment data serialization.

Closes-Bug: #1272349

Change-Id: I80d3ccdcc8e4b440f40f5b6e089532adaa5d02df
Merge "Added selective public network serialization"