Merge lp://staging/~abentley/ci-director/determine-cloud-health into lp://staging/ci-director

Implement pieces for determining whether a build's cloud was sickly.

This branch provides pieces that can be used to detect builds on sickly clouds.

It extracts get_substrate_from_parsed, so that it can be reused by get_health_match_data.

It provides get_health_match_data which describs which substrate and time window are relevant to determining whether the last build was sickly. It also provides a set of days mentioned.

load_cloud_health_data takes a set of days (i.e. from get_health_match_data) and returns a dict containing the relevant results.

find_cloud_health_builds takes a dict of cloud health data (i.e. from load_cloud_health) and finds the builds that match a given substrate and time window (i.e. from get_health_match_data).