Pecan WSGI: prevent plugins from opening AMQP connections

Pecan WSGI: prevent plugins from opening AMQP connections

The Pecan WSGI server is supposed to server REST requests only.
As several plugins initialize their RPC interfaces upon initialization,
this results in establishing undesired connections to the AMQP server.

This patch does not alter plugin behavior or initialization process,
but ensures that the setup_rpc operation performed by a plugin has no
effect when executed in the Pecan WSGI server.

This patch also performs some refactoring for server launchers.

Change-Id: I56384f5f964ea90d72babf911aa5639989e9c3d8