Check requirements.txt files for missing (used) requirements

Invoke the pip-missing-reqs tool to find packages used by nova but
not explicitly included in its requirements.txt file.

Add simplejson to the requirements to fix the failing check.

Change-Id: Ia90416c613c0acea04f3d4ab4898f550cb702a49
Merge "Check requirements.txt files for missing (used) requirements"