Add simple note about attaching neutron networks

I managed to follow the documentation properly for this example, however
something that was not clear to me was which networks attached inside
your VM.  So now, we display a little note hoping to help future users.

Also rename file from *pertenant* to *per-tenant*.

Change-Id: I0c4cbf9ae2ebb27037cbcc3b6cdf87069f69a16c
Signed-off-by: Paul Belanger <paul.belanger@polybeacon.com>