Enforce reverse time-order for sample return

Implements: blueprint api-sample-sorted

Added a new test test_get_samples_in_default_order() for class RawSampleTest in
tests/storage/base.py to check that the samples are sorted by timestamp, from
newest to oldest, e.g. first sample is newest

Added more samples to the test database in tests/storage/base.py , to make
testing of time ordering more robust, and appropriately changed tests to match
the expected results from the new modified test database.

Added code to sort samples in reverse time order, in get_samples() of
ceilometer/storage/impl_mongodb.py and ceilometer/storage/impl_sqlalchemy.py

Change-Id: Ib822bbbfc8fd47cf6685926762295577b69fd417
Merge "Enforce reverse time-order for sample return"