Add scatter gather utilities for cells

This adds utilities: scatter_gather_all_cells(),
scatter_gather_skip_cell0(), and scatter_gather_cells() for querying
cells in parallel using eventlet green threads.

Change-Id: I289932176e8029b0f9a76dbfb963f8ac218fdc06

nova/test.py

@@ -265,6 +265,7 @@ class TestCase(testtools.TestCase):
         from nova.compute import api
         api.CELLS = []
         context.CELL_CACHE = {}
+        context.CELLS = []
 
         self.cell_mappings = {}
         self.host_mappings = {}