Modify the OVS agent to support multiple tunnel_types concurrently.

This change modifies the "tunnel_type" configuration option to be a ListOpt
instead of a StrOpt. This is intended to allow the agent to work in an ML2
environment where multiple, disparate "tunnel_types" are supported at the
same time. For now, the agent will enforce a single "tunnel_type".

Fixes bug #1191089

Change-Id: Ic025e54a8a3e92fe1fe44ebf08d71c2210c7d82b
Merge "Modify the OVS agent to support multiple tunnel_types concurrently."