Refactor mlnx mechanism driver to support infiniband only

Refactor mlnx mechanism driver to support infiniband only

The old mlnx mechanism driver was used for SR-IOV with Ethernet
and infiniband support but the PCI allocation wasn't done by nova.
Juno introduced sriovnicswitch mechanism driver for SR-IOV with
Ethernet support. Mellanox recommends on using it.
The updated mlnx mechanism driver supports SR-IOV infiniband.

* support only port vnic_type 'direct'
* update vif_type name to ib_hostdev (the ib_hostdev is generic vif_type
  for SR-IOV infiniband see https://review.openstack.org/#/c/187052/)

Closes-Bug: #1460430
DocImpact

Change-Id: Ia822b492afdfeb1aecf373d5a8cdb72174512884