Ensure Flask uses our JSON Encoder

Since the upgrade to Flask 0.10, we've not been using the correct
JSONEncoder. Up until today, we this was "not a problem", as
python-keystoneclient depended on simplejson, which provided the
necessary features.

python-keystoneclient 0.4.2 was released today, causing the issue
to finally surface.

Change-Id: Idc0e8f03608c80e097db2ea48b30f2bc0b8dbe5d
Closes-Bug: 1268607