Improve the timestamp validation of ceilometer API

Currently, ceilometer API is lack of timestamp validation. Timestamp in
query should be checked if it can be parsed to iso timestamp, and raise
explicit exception if not.

Change-Id: I02cb2f30b00bde46bf8a0e4e161a0e2ce9d44c09
Closes-bug: #1270394
Merge "Improve the timestamp validation of ceilometer API"