service: use deny-list instead of allow-list for BEEFY (#5331)

* use deny-list instead of allow-list for BEEFY

Instead of allowing BEEFY to run on specific test nets,
inverse the condition to explicitly disallow BEEFY on
production chains that we don't want it to run on yet.

This allows other test chains (other than Rococo/Wococo)
that use the polkadot service file to enable and test BEEFY.

Signed-off-by: Adrian Catangiu <adrian@parity.io>

* address review comments

* throw error if BEEFY enabled on production networks

Signed-off-by: acatangiu <adrian@parity.io>