The 'check_exists' applied to all API calls

* 404 is now returned instead of 500 in all API calls;
* direct calls to nova client moved from api/v10 to service/api
* utils/api has been updated to avoid implicit magic.

Partailly implements blueprint savanna-rest-api-1-0-validation

Change-Id: Id1e64b03920c59bc96da6c8f709bd1beccb59d76
Merge "The 'check_exists' applied to all API calls"