add more test cases to improve the test code coverage #2

In this patch, the following modules test coverage improved to 100%

1. ceilometer/storage/sqlalchemy/models

Change-Id: Ie8a12ea07cb6c406f82cb29e8743fe67110c94c8
Add example with return values in API v2 docs

Add a "real" example of a groupby statistics query with return
values in API v2 documentation.

The example is taken from an actual query run on a machine
running Devstack and Ceilometer.

Addresses bug # 1228465

Change-Id: I0414fdaf31ca365f0404c77504d4106de7c17f74
Merge "add more test cases to improve the test code coverage #2"
Merge "Add example with return values in API v2 docs"