Add run modes (capture/proxy, only simulate)

[eduardongarcia]

Is your feature request related to a problem? Please describe.
When using tshield as mock-server it is desirable to disable the option to access external service in some scenarios.

Describe the solution you'd like
It would be nice to have some modes to run tshield:

• Normal mode: As it is today, if tshield does not have the requested resource, accesses the external service, writes the resource and returns it to the client, from the second call no longer accesses the external service for that match.

• Simulation mode only: tshield responds only to previously recorded resources, if it does not have the requested resource no request is made to the external service

• Fallback Mode: Always tries to access the external service and records the response on success, in case of communication failure with the external service returns the previously recorded resource.