Add NIC to instance: validation problems with IP address field

[askfongjojo]

Two issues here:

1. When adding a network interface to an existing instance, user can simply specify the VPC and subnet without a specific IP address. However, the address k/v pair is still passed to the API call with an empty value, resulting in an unhandled 400 error (i.e. the operation failed silently and the popup remains open).
2. If an IP address is specified but is invalid (e.g. outside of the subnet range), the Nexus validation error is also unhandled.

[just-be-dev]

Nexus lists the IP as nullable which is why it's optional in console. If it's required we should make an API update to reflect that.

[augustuswm]

Ran into this just now. Looking at Nexus it looks like it is intentional that the ip address is optional. I think the bug here is the the console is sending an empty string value when it should be omitting it.

The request sent on submission looks like:

{"name":"net0","description":"","ip":"","subnet_name":"default","vpc_name":"default"}

and the response is:

{
  "request_id": "6f39a0ab-a4eb-4904-bc42-9b1e8911e9e2",
  "message": "unable to parse JSON body: ip: invalid IP address syntax at line 1 column 39"
}

[david-crespo]

This wasn't fully completely by #1811.