Change the commit we're using for keystone.

mitaka doesn't seem to work; @henn suggested this tag instead.

ci/keystone/install.sh

@@ -2,4 +2,7 @@
 set -ex
 pip install keystonemiddleware
 pip install python-keystoneclient
-keystone_commit=stable/mitaka ./ci/keystone/keystone.sh setup
+# The exact commit we use here is somewhat arbitrary, but we want
+# something that (a) won't change out from under our feet, and (b)
+# works with our existing tests.
+keystone_commit=10.0.0.0b2 ./ci/keystone/keystone.sh setup