Add a transport options from client to drivers

Per conversation with Hervé Beraud, Ken Giusti and Ben Nemec.

New feature: Transport Options
With this feature, it is possible to pass parameters to the drivers so that we can use some specific driver options/features.

The idea is to create a generic "transport options", and then each driver will convert from generic to a specific option.

One example is the mandatory flag in RabbitMQ (currently not used, and there is no way to set it).

something like:
```
transport_options = oslo_messaging.get_transport_options(transport, at-least-once=True)
oslo_messaging.RPCClient(transport, target, transport_options)
```

The RabbitMQ driver implementation will convert:
at-least-once -----> mandatory=True

Since this feature requires several changes, we split it into three different steps:
1- Add a new parameter called "transport_options" from client to _drivers
2- Implement the drivers decode functions
3- Implement mandatory flag for RabbitMQ

For the other drivers, we will open a new issue.

-
Gabriele Santomaggio
Developer @SUSE