fixes path handling and cache

[sehnem]

This PR fixes a bad syntax that I sent in the previous PR and other two bugs.

• The fs did not work for paths starting with /, and was not consistent with other filesystems (s3, gcs and azure);
• In some cases the cached path was not saved starting with / and it was causing issues with the _ls_from_cache;

[sehnem]

@martindurant I made some changes to make the service_account option more inline with the gcs file storage, can you review the changes and let me know what I need to fix in it? Thanks!

[martindurant]

Please continue to ping me here until I review - things are pretty busy right now