rapi: Add new user option for querying

This was requested in issue 301. Before this patch, requests to
“/2/query/*” and “/2/instances/*/console” would require authentication
with a user with write access. Since that is not strictly necessary, a
new user option named “read” is added.

Console information can also be retrieved as a normal query, therefore
the change applies there too.

This was the first user option to be added after “write”, therefore
quite a few changes were necessary. Documentation, including NEWS, is
updated as well.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>