CLI improvements & fixes

[cecton]

These are a few changes I missed during the refactoring.

1. Initialization issue and boilerplate

   Most importantly: part of the Configuration initialization was done in sc_cli::init. This means the user can not benefit from this initialization boilerplate if they have multiple Configuration since sc_cli::init can only be called once.

2. Boilerplate for VersionInfo and Configuration

   I'm also answering to the critic of @bkchr on the initialization using version: https://github.com/paritytech/substrate/pull/4692/files/bea809d4c14a2ede953227ac885e3b3f9771c548#r372047238 This will allow initializing a Configuration and provide the version by default.

3. Loading the chain_spec explicitly

   In the past it was done automatically but in some cases we want to delay this. I moved the code to Configuration.load_spec() so it can be called later on. chain_spec can also be written directly to the Configuration without using this load_spec helper.

4. Make parse_address public

   This change is necessary to allow the user to change the default addresses of rpc_http, rpc_ws and grafana_port.

   [edit: I don't need that!]

5. Fixing issue that prevents the user to override the port

   In the refactoring I introduced a bug by mistake that could potentially prevent the CLI user to override the ports if defaults where provided for these ports (only on cumulus).

6. Change task_executor from Box to Arc

   This is useful for cumulus where we have 2 nodes with 2 separate Configuration that need to spawn tasks to the same runtime.

7. Renamed TasksExecutorRequired to TaskExecutor

   For consistency.

This is related to paritytech/cumulus#24

This is the continuation (and hopefully the end of) #4692

[parity-cla-bot]

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