rococo-runtime: `RococoGenesisExt` removed (#1490)

[`RococoGenesisExt`](https://github.com/paritytech/polkadot-sdk/blob/a414ea75

/polkadot/node/service/src/chain_spec.rs#L152-L171)
is removed. It was the hack to allow overwriting
`EpochDurationInBlocks`. Removal of `RococGenesisExt` prevents from
manipulating the state to change the runtime constants.

Changes:
- Environment variable which controls the `time::EpochDurationInBlocks`
value was added: `ROCOCO_EPOCH_DURATION` (epoch duration will be set to
the value of env),
- `10,100,600` versions of rococo-runtime are built in CI and put into `polkadot-debug` docker image.

`rococo-runtime` building examples:
- to build runtime for `versi_staging_testnet` which had
EpochDurationInBlocks set to 100:
  ```
ROCOCO_EPOCH_DURATION=100 cargo build --features=fast-runtime -p
rococo-runtime
  ```
- to build runtime for `wococo_development`
  ```
ROCOCO_EPOCH_DURATION=10 cargo build --features=fast-runtime -p
rococo-runtime
  ```
- to build `versi-staging` chain spec:
  ```
ROCOCO_EPOCH_DURATION=100 cargo run -p polkadot --features=fast-runtime
-- build-spec --chain versi-staging --raw
  ```
- to build `wococo-dev` chain spec:
  ```
ROCOCO_EPOCH_DURATION=10 cargo run -p polkadot --features=fast-runtime
-- build-spec --chain wococo-dev --raw
  ```

It is also possible to change the epoch duration by replacing the `code` field in the chain spec with the hex dump of pre-built runtime wasm blob (because the epoch duration is hard-coded into wasm blob).

---------

Co-authored-by: Bastian Köcher <git@kchr.de>