Add endpoint for hypervisor stats.
user
```ruby
module Fog
  module Compute
    class OpenStack
      class Real
        def get_hypervisor_statistics(tenant_id)
          request(
            :expects  => 200,
            :method   => 'GET',
            :path     => "/#{tenant_id}​/os-hypervisors/statistics"
          )
        end
      end

      class Mock
        def get_hypervisor_statistics(tenant_id)
          response = Excon::Response.new
          response.status = 200
          response.body = {
            "hypervisor_statistics" => {
              "count" => 1,
              "current_workload" => 0,
              "disk_available_least" => 0,
              "free_disk_gb" => 1028,
              "free_ram_mb" => 7680,
              "local_gb" => 1028,
              "local_gb_used" => 0,
              "memory_mb" => 8192,
              "memory_mb_used" => 512,
              "running_vms" => 0,
              "vcpus" => 1,
              "vcpus_used" => 0
            }
          }
          response
        end
      end
    end
  end
end
```
assistant