Don't accept unix: connections, etc. in OpenFlow controller discovery.

Don't accept unix: connections, etc. in OpenFlow controller discovery.

The controller discovery code has always had the capability to whitelist
only certain types of controller locations.  Until now, we have only taken
advantage of this when SSL is enabled (so that all OpenFlow connections are
authenticated with SSL if SSL is configured).

However, it occurs to me that making the section of connections entirely
unrestricted is too permissive.  An attacker could make the vswitch connect
to an arbitrary Unix domain socket, for example.  I don't have a
description of how this is an exploitable security vulnerability, but it
seems entirely too lax.

So: this commit changes the default to allowing only TCP connections to
controller in the non-SSL case.