Revert some of the PR changes to tests to make sure that all of the
old format function invocations work with the old and new tests.
Implement small enchancements to documentation and credentials resolving
for usability and flexibility with custom authentication backends.
Update documentation to indicate that backwards compatibility
is supported as well as the new format credentials invocations.
Signed-off-by: Aleksi Häkli <aleksi.hakli@iki.fi>
