Add webhook callbacks usage

[arbulu89]

Implement the check results execution callbacks system.

• Remove the api-host and api-port cmd flags and use the callbacks-url flag, which will have the trento server callbacks url (/api/runner/callbacks or something, but it will be configurable)
• Add the callbacks client code, which sends POST messages to the callbacks url with the predefined format (execution_id, event and payload)
• Add first callback usage to notify an execution start even

Next things:

• Implement the whole execution request data structure and the real execution itself
• Update the python ansible callback to send a execution_completed callback
• Update the execution completed callback payload format