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
```
The goal is to allow in a future version, to pass in other keys to configure events collection, say scope or filters (for now it's hardcoded see https://github.com/DataDog/dd-agent/blob/leo/vmwareremovealerts/checks.d/vsphere.py#L37-L56)