Add index for db.meter by descending timestamp

Fixes bug #1193906

Added an index for db.meter to sort by descending time stamp for class
Connection in ceilometer/storage/impl_mongodb.py

ensure_index() is used rather than create_index() to be more efficient.

This index is needed because get_samples() in
ceilometer/storage/impl_mongodb.py sorts by descending timestamp.  Without an
index, MongoDB will fail to sort if the database is too large.

Change-Id: I23070f153ce03a8d6d8c9f17f05e2d5dae38647e
(cherry picked from commit e984f28b00e3a073c176f6435507f5bce1c82d30)
Merge "Add index for db.meter by descending timestamp" into stable/grizzly