Adds v3 API samples for limits and simple tenant usage

Adds the v3 API samples for the limits and simple tenant
usage extensions. Bundled together to reduce review load

Partially implements blueprint v3-api-specification

+DocImpact

Change-Id: I1b73d7acf8d1bbccec2fff6180799edd3f3b12aa
Add limits,simple-tenant-usage metafiles for api samples

Converts limits,simple-tenant-usage api sample to create meta
files for api sample docs. We use this to auto generate the
specification document.

Change-Id: Ibecfe8c28d78a9c23edd9b9512d84fe5db29e37b
Merge pull request #15 from soulxu/add_metafiles_limits

Add limits,simple-tenant-usage metafiles for api samples