Introduce Wococo testnet (#2918)

* runtime: remove mmr and beefy from westend runtime

* runtime: westend: remove pallet_beefy config

* node: only start beefy gadget on rococo

* node: remove beefy keys from westend chain spec

* node: add wococo testnet chain spec

* node: add comments about beefy gadget task

* runtime: update wococo chainspec

* Remove stale comment

Co-authored-by: Tomasz Drwięga <tomasz@parity.io>
Co-authored-by: adoerr <0xad@gmx.net>