Description of problem:
When adding a router interface to network, the interface should get the first free IP address from the subnet (/24).

Version-Release number of selected component (if applicable):


How reproducible:
100%

Steps to Reproduce:
1. create router "test_router"
2. create network "test_net"
3. in test_net create subnet "test_subnet":
net address 10.0.0.0/24
IPv4
checkDisable gateway
click to Create
4. Network topology, click to router, Add interface, select "test_net: 10.0.0.0/24 (test_subnet)", leave IP address empty, submit the dialogue
5. red popoup is displayed: "Error: Failed to add_interface: Bad router request: Subnet for router interface must have a gateway IP" - it's valid behaviour so far
6. networks - click to "test_net" row, edit subnet "test_subnet":
uncheck Disable gateway,
set gateway IP 10.0.0.1,
check enable DHCP (is already checked by default),
allocation pool: 10.0.0.2,10.0.0.254
hit Save button
7. Network topology, click to router "test_router", Add interface, select "test_net: 10.0.0.0/24 (test_subnet)", leave IP address empty, submit the dialogue


Actual results:
Adding interface is trying to use first IP address from the subnet range, but it's already used as gateway, so red popup is displayed instead:
Error: Failed to add_interface: Unable to complete operation for network 490131be-13e9-49b8-b515-6d2dec8847da. The IP address 10.0.0.1 is in use.

Expected results:
The first unoccupied address is selected automatically.

Additional info:
If is new subnet created with gateway IP from beginning, router port is created successfully and it picks the same IP as is defined as subnet gateway