Remove alarms by default from vCenter

I think they're a bit spammy, so they should probably be disabled by default, what do you think @remh ?

To enable them again:
```yaml
instances:
    - name: blah
      event_config:
        vcenter_alarms: true
```
Is it worth passing a dictionary ?
It looks like you are just using one single key in that dictionary: collect_vcenter_alarms