Standardize timestamp fields of ceilometer API

For currently, the timestamp in query fields is not unified, this
patch standardizes the usage of timestamp query fileds:

- unify the time query filed in API to *timestamp* supported only.
- unify the time related fileds of filters to start_timestamp and
  end_timestamp to simplify the process logic.
- improve the _query_to_kwargs method to give explicit presentation
  if user specifies unsupported query fields.

Change-Id: Ic3ade312efa89edd01d2ee5ae31d341805b11f79
Closes-bug: #1295104
Closes-bug: #1291171
Merge "Standardize timestamp fields of ceilometer API"