Move camera credentials from .env to credentials.json

[fe51]

Pitch

Thanks to #312, camera management logic has been generalized.
However, the credentials for Reolink cameras are still not defined in a standardized way.

What is done currently

Reolink camera credentials are still stored in .env file

pyro-engine/README.md

Line 71 in a0825ac

- `CAM_USER`: the username to access the camera

pyro-engine/README.md

Line 72 in a0825ac

- `CAM_PWD`: the password to access the camera

What should be done

• Updates the code logic to have these camera credentials defined in credentials.json
  (there will be some udpates in the Engine logic)
• Updates readme consequently

Happy to discuss it :)