[functest] Update tests for Aodh

[functest] Update tests for Aodh

* Update the check for timestamp to use state_timestamp instead
  state_timestamp indicates the last change in state of the alarm,
  timestamp is the last update to the alarm settings. This is not
  what we're looking for with aodh tests, so check the state update
  instead.
* Trigger an alarm state change before checking whether the alram
  was updated. Notification in collectd aren't necessarily updated
  at a regular interval, so we have to do some action and check whether
  the alarm is updated.

JIRA: BAROMETER-91
Change-Id: I26d9bb1ec7e50e07053e69068d241e4c4cbc30dc
Signed-off-by: Emma Foley <emma.l.foley@intel.com>