Include documentation for IOTA env vars (as in other IOTAs)

[fgalan]

Env vars used by this IOTA has not been documented. We should use the same documentation approach than in other IOTAs.

At the present moment, there is only one variable: IOTA_SIGFOX_PORT (see https://github.com/telefonicaid/sigfox-iotagent/blob/master/lib/configService.js)