Port pagination on Python 3

Use an empty string rather None as marker for bisect.
On Python 3, a comparison between None and strings now raises a TypeError.

Partially implements: blueprint trove-python3

Change-Id: Iaefc20bd1cc980732e0c8a075e570958335c17e1

blacklist-py3.txt

@@ -1,6 +1,4 @@
 # Use a blacklist of tests known to fail on Python 3, until
 # all unit tests will pass on Python 3.
-guestagent.test_cassandra_manager
-guestagent.test_mongodb_manager
 guestagent.test_operating_system
 guestagent.test_volume

trove/common/pagination.py

@@ -37,6 +37,8 @@ def paginate_list(li, limit=None, marker=None, include_marker=False,
     """
     sli = sorted(li, key=key)
     index = [key(item) for item in sli]
+    if marker is None:
+        marker = ''
     if include_marker:
         pos = bisect.bisect_left(index, marker)
     else: