Description of problem:
[Default route] - Engine allows to set network as default route role without DG, but complains about it.

Engine allow the user/admin to set a network with a default route role with only IPv4/IPv6 address + subnet/prefix, but without default gateway, which is actually required for the default route role. 

Engine doesn't validate the default gateway, but it does complains about it and show the network as out-of-sync because of default route property, which is also misleading. 

Version-Release number of selected component (if applicable):
4.3.0-0.8.rc2.el7

How reproducible:
100%

Steps to Reproduce:
1. Set a non-mgmt network with default route role
2. Attach the network to the host with only IPv4 or IPv6 address and subnet. Without default gateway

Actual results:
Network is out-of-sync

Expected results:
1. Or validate that network has a default gateway if it's a default route role
2. Or don't complain about it, don't show the network as out-of-sync

Additional info:
Default gateway is required for default route role, for both IPv4 and IPv6.