Do not paginate stack list unless page_size is set

Pagination on the server has not yet been implemented, however
the client assumes it is, causing infinite recursion when paginate
is called.

With this change, no pagination is attempted unless page_size is
specified. However the 'limit' parameter is still honored, so it
is still possible to cap the total number of results returned.

heatclient users should not set page_size until pagination has been
implemented on the server. The stack list in Horizon currently
does not attempt to paginate, so is not affected.

Fixes: bug #1207839
Change-Id: I4087d3a8af48206d6ebe3edc441469464e4a401a