[digitalocean|compute] added create_ssh_key mock code

lib/fog/digitalocean/requests/compute/create_ssh_key.rb

@@ -17,7 +17,19 @@ module Fog
       class Mock
 
         def create_ssh_key( name, pub_key )
-          Fog::Mock.not_implemented
+          response = Excon::Response.new
+          response.status = 200
+          mock_data = {
+            "id" => Fog::Mock.random_numbers(1).to_i,
+            "name" => name,
+            "ssh_pub_key" => pub_key
+          }
+          response.body = {
+            "status" => "OK",
+            "ssh_key"  => mock_data
+          }
+          self.data[:ssh_keys] << mock_data
+          response
         end
 
       end