The location property of the glance_image type has nearly identical
functionality as the source param, yet one is a param and one is a
property. It is possible to change either of these for a glance image,
but only if it is as of yet unset, and therefore unsaved. It is not
possible to change the location of a glance image once it has been
saved, nor is it possible to query it afterward. Since having location
be a property when it is usually unsettable and is always unqueryable
does not make much sense, we change it to be a param to be consistent
with the source param.
Change-Id: I686c3fec9bf571d18e282888c626d795c9958a6b