Remove __init__ method from TunnelCallback mixin

Removed an __init__ method from a mixin class that
made mixing with other classes fragile and inflexible.
This replaces it with an explicit setup method.

This allows the ML2 RPCCallbacks class to correctly
inherit the common RpcCallback class.

Closes-Bug: #1332041
Change-Id: I36cb7dcf57147468f252d61f846b2b28dd77c8ff
Merge "Remove __init__ method from TunnelCallback mixin"