implement gerrit pagination in simplequery

gerrit defaults to a limit of 500 changes returned. Nova regularly has
more than 500 open reviews. This means that some operations aren't
seeing all the open reviews, and causing some oddness (like merge
check).

Implement implicit paging.

In looking at this code I also believe I found a double -1 slicing
error which would mean that we would miss some reviews being listed.

Change-Id: I00d47c05e4a740b61c690510593fd29eaef49679
Merge "implement gerrit pagination in simplequery"