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 D3832.