Currently, we have 4 types of arguments in LUXI calls:

- most common, a list of values
- a single argument that is sent as a list of one element
- a single argument that is sent by itself
- a dictionary (only Query and QueryFields)

This inconsistency makes it not only harder to auto-generate the
HTools LUXI interface, but also in general to check the arguments and
(if we ever want to do it) auto-generate the Python LUXI client.

Compare this with the node daemon, which uses consistently a list for
its arguments, and even with way more changes over time had no issues
with extending the interface.

In case we want to extend a call, there are two options:

- preferred: add a new call, keep the old one unchanged
- possible: add further parameters to the current argument list

The patch against HTools will follow—sending separately as the Python
changes are very clear by themselves.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>