net/mqtt: Simplify the MQTT high-level API

1) Remove some variables pointing to user-provided data.
2) Pass the context structure instead of those variables.
3) Homogenize the use of "ctx" for all the callbacks receiving the
   struct mqtt_ctx * pointer.

Now users must use the CONTAINER_OF macro to access data required
by the MQTT callbacks.

Change-Id: I871c0bd8601a67b39187683215579f9ed0087cf9
Signed-off-by: Flavio Santes <flavio.santes@intel.com>