Set network_mode to bridge to overwrite old setting

Previously, the network_mode was only set when creating a job with
network_access = false. This results in Nomad leaving this setting
as is when updating the job to use network. Thus a job would have
had the mapped ports in the Nomad UI, but the Docker network_mode
would still be 'none'.

environment/job_test.go

@@ -192,7 +192,8 @@ func TestConfigureNetworkSetsCorrectValues(t *testing.T) {
 			}
 
 			mode, ok := testTask.Config["network_mode"]
-			assert.False(t, ok && mode == "none")
+			assert.True(t, ok)
+			assert.Equal(t, mode, "")
 		}
 	})
 }