Now is possible to define a gateway IP when creating a subnet using a
subnet pool. The IPAM subnet generator retrieves the available IP
ranges in the subnet pool and generates a list of candidate subnets
with the prefix lenght defined. If the gateway IP can be allocated in
one of those candidate subnets, the IPAM returns a valid IpamSubnet
that will be used to create a Neutron subnet.
Closes-Bug: #1904436
Change-Id: Ib1d1f591c4d0f59ebff3ddcb3be7b10b0b5e67dc