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
Introduce bulk calls for get device details

Allow to get multiple devices details instead of just one
This change introduces a new method in the rpc api.

blueprint bulk-get-device-details

Change-Id: I8497256d7f4f2fb48b5cb792e35aaedf63f129fc
Merge "Remove __init__ method from TunnelCallback mixin"
Merge "Introduce bulk calls for get device details"