OS Ceilometer API support

OpenStack Ceilometer API's support (Skeleton & meters/samples API)


open issues I need help with -
1. meters and samples can get query params, problem is they cant be sent as null/empty string since ceilometer fails. example - {"filter": "", "orderby": "", "limit": ""}
need a way to avoid sending empty query params 
2. for meters API -
currently I've created "MetersApi.listMeters(QueryOptions... options)" should I follow this notion or simplify it as
"MetersApi.listMeters(String filter)" where filter is a json filter as in item #1
3. note that MetersApiMockTest.testListMetersWithQuery is failing since I cant get the API to send the QueryOptions on POST, it always goes on the GET as query params. any help would be appritiated 
Please annotate the API interfaces in this PR with `@Beta`