Add ming as a test requirement

Ming is an ORM layer for Mongo. The package also
includes MIM, and in-memory MongoDB-compatible
driver. Ceilometer uses MIM for testing.

Addresses bug 1176017

Change-Id: I8882ffc7ecf9306acc4cc8bc21ccdb84718d8883
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>