Update neutron ML2 configuration

I updated the installation guide neutron ML2 configuration to
use the newer '[agent] tunnel_types' option instead of the
'[ovs] tunnel_type' option. I also improved the description of
a few steps.

Change-Id: I9ddc150b7f09b15afaf1b9a4113a0fb8b13fe032
Closes-Bug: #1391097
backport: juno
(cherry picked from commit 300aaf719d6088096d0914df4375384f930a9605)