Refactor timestamp existence validation in V2 API

There was a bug in AlarmThresholdRule in v2.py, the timestamp_keys
were defined with a missing comma:
['timestamp', 'start', 'start_timestamp' 'end', 'end_timestamp'].

The validation, if the timestamp field is used or not, was unclear
in the current implementation. A new parameter, 'timestamp_is_valid',
was added to the validation functions to show, if the timestamp is
allowed to use or not in the validated query.

Fixes bug 1280975

(cherry picked from commit b1834e55fce08f1536c7d9392ae035b9583be8ed)

Change-Id: I2aaef54354926f9afe3bb1d4fae8f8aa0ae600ab
Merge "Refactor timestamp existence validation in V2 API" into stable/havana