Issue #403 adding galera alarm

Adding first alarm for maas check : galera_check
This looks like a fine addition to the checks however I would like to see the check's constraint be changed to the count of galera nodes as found within the clusters inventory.  

``` yaml
alarms:
  - { 'name': 'galera_cluster_status', 'criteria': ':set consecutiveCount={{ maas_alarm_local_consecutive_count }} if (metric["WSREP_CLUSTER_SIZE"] < {{ groups["galera"] | length }}) { return new AlarmStatus(CRITICAL, "Galera Cluster degraded"); }' }
```