Refactor to use DSC for installing the service

* Add new parameter `$windows_service_user` which can be used to run the
  service as a different user.
* Use `dsc_service` resource instead of `exec` to install the service.
  If `$windows_service_user` isn't set then this will just use the Local
  System user as normal.
* If `$windows_service_user` is set then also use the
  `dsc_userrightsassignment` resource to grant the "Log on as a service"
  right to the user and also use the `acl` module to grant permission
  for the user to access the Sensu install directory.

Fixed tests with new Windows modules

added spec tests

Readded powershell dependency (needed by dsc)

Powershll module already installed by dsc, avoid version conflicts

…ll 5+

Reload action in vagrant requires "vagrant plugin install vagrant-reload"

…resent

Dsc (continued)