Support some rare case of get_one()

There is some special case for get_one usgae, for i.e., Ceilometer
disable /v2/event_type/(event_type), so user GET that API will return
404 error. But this url should be exposed to pecan, so
/v2/event_type/(event_type)/traits url can be routed. In such
situation, get_one() will not have wsme_definition.

Currently, we support /v2/event_type/(event_type)/(trait_name), which
means, TraitController has __init__(self, event_type) and get_one(self,
traint_name). But what if /v2/event_type/(event_type)/traits/(trait_name)
is what api want? This patch adds such support by check arguments of
get_one(), and filter out argument.name which already in webprefix.

Related-Bug: #1388514