lister: Unify credentials configuration structure between listers

This becomes a dictionary of key , value a dict of key , value list of dict username/password.

This allows configuration change to allow the following:

  credentials:
    github:  # github lister
      github:  # has only one instance (so far)
      - username: some
        password: somekey
      - username: one
        password: onekey
      - ...
    gitlab:  # gitlab lister
      riseup:  # has many instances
      - username: someone
        password: ...
      - ...
      gitlab:
      - username: someone
        password: ...
      - ...

Related swh/infra/puppet/puppet-swh-site#1772 (closed)

Test Plan

tox

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Migrated from D1518 (view on Phabricator)