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
Add docs about volume/snapshot measurements

This change allows to add information about
measurements of volumes/snapshots.

Implements: blueprint add-new-notifications-types-for-volumes-and-snapshots
Change-Id: I23ec262dc4617af7c270ffc1a4dd17ad9ac69841
Make compute discovery pollster-based, not agent-level

Partially-Implements-Blueprint: merge-compute-central-agents

Change-Id: I02c9a12b0fcd55a7d59688853f3c08e505b4bf3f
Merge "Add docs about volume/snapshot measurements"
Merge "Standardize timestamp fields of ceilometer API"
Merge "Make compute discovery pollster-based, not agent-level"