Standalone NSFS support account auth with access_key/secret_key and mapping to uid/gid

[guymguym]

Environment info

• NooBaa Version: master-20210426
• Platform: Standalone
• CC: @eshelmarc @jeniawhite

Actual behavior

1. Currently the standalone noobaa-core does not do any access authentication.
2. Also there is no mapping to uid/gid.

Expected behavior

1. First step is to allow a global account for the standalone endpoint which will have access_key/secret_key.
2. By default it should map to the current user uid/gid.
3. Second step is to allow a separate account for every bucket, with its own uid/gid mapping.

Steps to reproduce

1. NA

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