Add 'no_data_policy' attribute to alarm definition

https://review.openstack.org/359228
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.

commit 16b4b02d9eae95ecf683ff6b52b6d54a95a0d232
Author: Swann Croiset <email address hidden>
Date: Tue Aug 23 16:10:23 2016 +0200

    Add 'no_data_policy' attribute to alarm definition

    This allows to specify the behavior of the alarm when no datapoint have
    been received in the defined window. The default value is to send an AFD
    alert with 'UNKNOWN' to be backward-compatible with the existing alarms.

    The value for the 'no_data_policy' attribute can either be a severity
    level (okay, warning, critical, down, unknown) or 'skip' to avoid
    sending the AFD metric.

    This change is required to define relevant alarms against "sporadic"
    metrics which aren't guaranteed to be received at periodic intervals
    like for instance "resource creation time" metrics. It also needed to
    support alarm definitions for cluster-wide metrics.

    DocImpact

    Change-Id: Iaa7fe63d321403ac2c5d4695a3346d912d3cc984