Hi Vincenzo,
what do you mean for "virtualize the instance with infiniband"? Do you want to provide IB adapters to your instances via SR-IOV virtual functions? In the configuration I described above I have not set up SR-IOV virtual functions but I only use the Ethernet paravirtualization driver (eth_ipoib) on top of IPoIB devices. If you are interested in true Infiniband virtualization via SR-IOV virtual functions you should refer to these links:
Mellanox-Neutron-ML2 - OpenStack (see the Infiniband links)
Mellanox OFED Driver Installation and Configuration for SR-IOV
I didn't try it with Juno but last year I set up Openstack Havana with SR-IOV, so please feel free to contact me if you need help.
Thanks,
Ale