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
```
Also you shouldn't use mutable default arguments in general because it can cause bad bugs down the line, see: http://docs.python-guide.org/en/latest/writing/gotchas/#mutable-default-arguments