Models inserted in the world before enabling the physics do not move

[diegoferigo]

For some reason, the last physics alignment #365 has broken one of the tests. In short, in a world without physics, if:

1. A model is inserted
2. The simulator is stepped (there's no physics, the model does not move)
3. The physics plugin is inserted
4. The model is processed by the physics plugin

In these conditions, before #365 the model would be affected by the physics. Instead, after #365, the model stays still, like it would be static.

gym-ignition/tests/test_scenario/test_world.py

Lines 184 to 185 in 311b08e

	# The cube should start falling
	assert cube.base_position()[2] < cube_pose.position[2]

This is an edge case we were testing. For the moment, let's keep this behavior broken and disable the test.