Change UI descriptions of Mellanox components

This commit include changes for Mellanox UI:
  1. Change UI descriptions to better explain restrictions.
  2. Move number of virtual NICs beneath Mellanox SR-IOV
     plugin.
  3. Add opposite restrictions to openstack.yaml.

Closes-bug: #1346224

Change-Id: I2d7d218ac465e7be9e3b9650dc972051bde1b165
Signed-off-by: Aviram Bar-Haim <aviramb@mellanox.com>