This change enables to specify which fixed-ip will be added to a vm using:
openstack server add fixed ip <vm> <network> --fixed-ip-address <ip>
This change uses interface_attach instead of add_fixed_ip[1] which is
less flexible and uses a deprecated API.
[1] https://review.openstack.org/384261
Closes-Bug: #1678140
Change-Id: I7fe4621439ef0d8dca080551ffaeb614c5a91174