Add a per-transport allow_remote_exmods API

Currently we have a allowed_rpc_exception_modules configuration variable
which we use to configure a per-project list of modules which we will
allow exceptions to be instantiated from when deserializing remote
errors.

It makes no sense for this to be user configurable, instead the list of
modules should be set when you create a transport.

Closes-Bug: #1031719
Change-Id: Ib40e92cb920996ec5e8f63d6f2cbd88fd01a90f2