Remove alarms by default from vCenter

To enable them again:
instances:
    - name: blah
      event_config:
        vcenter_alarms: true
Rename new vsphere option to collect_vcenter_alarms
Don't use mutable default value in event_config
Initialize event_config on empty dict
Merge pull request #1143 from DataDog/leo/vmwareremovealerts

Remove alarms by default from vCenter