Integrate `mountpoint` health check with external monitoring systems like Zabbix (configurable)

depends on #1843, #1845

- [ ] Add an internal documentation (or update the installer) to configure an integrated monitoring system (e.g., Zabbix) to call the monitoring alarm API as defined in #1845.\* e.g., `POST http://{manager-addr}/monitor/alarm?reason=storage-unmounted&data={encoded-vfolder-host-dir-list}&src={agent-id}&token={some-preshared-key`}

- [ ] **manager:** Upon receiving an "storage-unmount" `MonitoringAlarmEvent` (refs #1845):\* Turn off the designated agent's `schedulable` attribute.
- Add the agent status history item (refs #1843) with the reason `"schedulable-off"`.
- [ ] Let's provide an admin option to automatically set `schedulable` on when recovery is detected. We could use "storage-remount" as the reason in the `MonitoringAlarmEevent`.

- [ ] **manager:** Add an etcd configuration key `config/manager/monitor/auto-exclude-agent-upon-storage-mount-failures` to enable/disable the above behavior.

JIRA Issue: BA-160