Adds POST method support in the metadata service

Fixes bug: #1092311

The password generation feature introduced in Grizzly requires POST
support in the metadata service.

Support has been added to both the metadata agent and namespace proxy.
Tests have been added and updated accordingly.

Change-Id: If7ef6bede1775ad682822cb868518ec732b90805
Let the cover venv run individual tests.

Also, delete cruft from tox.ini while we're in there.

Change-Id: I8fe43695bb25b9c5eec10711b12d2002f1237e52
Merge "Adds POST method support in the metadata service"
Merge "Let the cover venv run individual tests."