WIP: Create metafiles for API v3 samples

Create metafiles for the API v3 samples which can be used to
help automate the generation of specification documentation for
the v3 API

Adds the infrastructure for creating the meta files plus adds
the changes for hosts extension

Partially implements blueprint v3-api-specification

Change-Id: Ie443931ea6bb49fbf2848f2223d0e47436005c8f
Merge commit 'refs/changes/69/40169/4' of https://review.openstack.org/openstack/nova into metafile_base_update1
Adds os-agents metafiles for api samples

Converts os-agents api sample to create meta files for api sample docs.
We use this to auto generate the specification document