Correct alarm timestamp field in unittest code

This patch corrects some code in unit test which relates to timestamp
issue. timestamp and state_timestamp fields will always be set, but
test code sometimes sets them to None, which will affect source code
to consider the scenario timestamp is None, in fact, this would never
happen in API call.

This patch adds a type check for timestamp and state_timestamp
in alarm.storage.models.Alarm, to prevent contributors to accidently
set these two fields to None.

Change-Id: I035b4f0f9c1ff7f4158c72acfddaf02e55891086
Merge "Correct alarm timestamp field in unittest code"