snapshot import ignores "data-dir" field from --config-file option

When importing a snapshot using a data-dir set in a config file passed as --config-file foo.json, tezos-node ignores it and uses ~/.tezos-node.

One needs to explicitly pass the --data-dir option to choose where to import.

Two possible fixes:

• fix the automaton that decides priority between options, config-files and default values
• remove the --config-file option all together to remove the ambiguity (users would only have --data-dir available)