Implement the /v2/sample API

This is a simple implementation of the new sample endpoint in the API.
It provides raw accesses to the samples collected by Ceilometer, without
imposing any kind of filtering on the meter name.

The fields are also correctly named this time.

Change-Id: I9e01a082c04e0462811befd98899e2f2d4a4dc82
Blueprint: sample-api
api: raise ClientSideError rather than ValueError

This also enables the error message to be translatable.

Change-Id: If63ba0f7bc04eb490578c7598875407e796700aa