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
Requiring '==' is rarely the right thing to do.