Adds API samples and unitests for os-server-usage V3 extension

Partially implements blueprint v3-api-unittests

Change-Id: I21c9301f53e0e0d85b27a69592bf64eab046ff79
Add server-usage metafiles for api samples

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

Change-Id: I91c47205fe824e29622a961c5232f314c77e17ac
Merge pull request #16 from soulxu/add_samples_server_usage

Add server usage metafiles for api samples