Support delete datasource

Before Rocky, in order to change the datasources configuration (e.g. start/stop getting alarms from Zabbix), the user needed to modify /etc/vitrage/vitrage.conf and restart Vitrage.

In Rocky we are going to have full support for Vitrage graph persistency, so this will not be good enough. For example, if a user removes zabbix from the list of 'types' in vitrage.conf, it will not cause Zabbix alarms to be deleted.

Optional solutions:
1. Add an API for datasources configuration. Delete datasource will delete all its resources/alarms.
2. Add code in the consistency that handles this case.