More on Open Stack networking can be found here

There are times you may need separate IP addresses on an instance.  You can do this by attaching a new interface to your instance.

Remember: Each project is assigned a quota for managing storage and compute usage as well as IP addresses.  At this time, each project is allocated with 10 IP addresses.  If you require more than 10, please email ccr-help.  Additional costs may be associated with IP allocations over 10.  To avoid using more than one IP address on an instance, we encourage you to use virtual IPs if possible.  You can search the internet for documentation on IP aliasing or virtual interfaces for instructions.

Once you attached the network interface to your instance in the Openstack console, you will need to configure it in linux.  Below are instructions for doing so in Ubuntu 16.04.  Other versions of Ubuntu and RedHat/CentOS are similar but not exactly the same.  

Log into the machine and determine what network interfaces are in use:

Setup additional network interface:

cd /etc/network/interfaces.d

vi ens4.cfg