High Availability capability additions for AlertManager

From the initial investigation done in #81 (closed)

The following lacking areas were found to support Alertmanager High Availability out of the box:

• Alertmanager HA internal discovery

  • The value prometheusOperator.clusterDomain must be uncommented

• Alert Manager communication with webbooks to MatterMost

  • add network policy conditional on mattermost value to allow alertmanager to egress to Mattermost

• Authservice enabled alongside monitoring.sso

  • authorization policies updated to ALLOW port 9094 of alertmanager so it isn't checking for Authorization for any requests (see other templates in chart/templates/bigbang/authorizationpolicies/)