Add a --check-config option for cli commands

this allows to specify on command line wether to check the configuration and read or write access to the storage at startup time (especially for rpc-server and replay commands.)

Warning: this option defaults to "read" for the backfill command and "write" for rpc-serve and replay commands, so now the creation of the Storage instance used in cli commands will be checked.

Also remove the deprecated config-path option from swh storage rpc-serve command and include the validation of the presence of a "storage" config section in the main storage click.group, where the config-file is actually parsed.

Depends on !550 (closed).

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Migrated from D3834 (view on Phabricator)