SCVMM scheduled EMS refresh.

A mechanism was needed to refresh the SCVMM provider given that event support will not be in the 3.1 release. I have configured a 4 minute schedule but am open to negotiation on the frequency. On one hand we need to bear in mind that an EMS refresh can take more than 4 minutes but on the other hand we can't wait too long after a VM power operation.
@bronaghs I think we need to queue this in the zone as each instance of EmsMicrosoft is associated with.  Also, the role should be `ems_inventory`, I believe, so that it lands on the appliance doing the refreshes.  So, I think you need to do something like:

```ruby
def ems_refresh_scvmm_timer
  EmsMicrosoft.each do |ems|
    queue_work(:class_name => 'EmsMicrosoft', :method_name => 'refresh_ems', :args => [ems.id], :zone => ems.zone.name, :role => 'ems_inventory')
  end
end
```