Allow opt-out of telemetry via environment variables and command-line args

See https://github.com/netdata/netdata/issues/7366#issuecomment-560169517 from @titpetric 
We need to make it simpler for people to opt-out from telemetry when installing via docker (and obviously document the fact) and add a CLI option to netdata. 

Related: https://github.com/netdata/netdata/issues/7401