To enable the possibility to migrate a non-routed network to a
routed network allow updating the segment_id of a subnet.
Only allow the operation if:
- The network only has one segment
- The network only has one subnet
- The current segment_id == None
APIImpact: The segment_id attribute of subnets now allows put operation.
Closes-Bug: #1692490
Depends-On: Iffda823a149a1143f46ee9a05e9640b34bf42c51
Change-Id: I1aee29dfb59e9769ec0f1cb1f5d2933bc5dc0dc5