Refactor timeout passing and standardize with keyword parameters

Standardize all HTTP request calls to use keyword parameters for all but
the URL. This makes the refactoring of including the timeout in these
requests' parameters easier and more uniform-looking. Tweaks to the
tests to comply with this new parameter passing scheme, in particular to
the API calls assertions.

Signed-off-by: Maxime Petazzoni <max@signalfuse.com>