python3: iteration order of dict is unpredictable

In Python 3.3, hash randomization is enabled by default. It causes the
iteration order of dicts and sets to be unpredictable and differ
across Python runs.

Sort the metadata.keys() in reverse order to keep the test cases as is.

Change-Id: I233ada4dae4e9c0bc97bf8fd7d912a0eff9dd5b8
Signed-off-by: Chuck Short <chuck.short@canonical.com>
Merge "python3: iteration order of dict is unpredictable"