I was able to reproduce this with an "all-in-one" devstack node
DevStack Version: queens Change: cc3750013b21d5c7011fc56effae14990efd4f9b Follow job renames 2018-09-12 09:35:47 +0000 OS Version: Ubuntu 16.04 xenial
When adding the interface it does in fact give it the gateway IP address.
I was able to reproduce this with an "all-in-one" devstack node
DevStack Version: queens 7011fc56effae14 990efd4f9b Follow job renames 2018-09-12 09:35:47 +0000
Change: cc3750013b21d5c
OS Version: Ubuntu 16.04 xenial
When adding the interface it does in fact give it the gateway IP address.