Fix support for PrismaticJoint in iDynTree::getRandomModel

[traversaro]

The actual method was fixed accidentally with a direct push to master (see 9c7f2b3), anyhow more fixes were necessary (for example, when passing noFixed argument no prismatic joint were generated), so this PR adds the necessary bits.

[traversaro]

Apparently InverseKinematics tests do not work with prismatic joints:

	 92 - UnitTestInverseKinematics (Failed)
	 93 - memcheck_UnitTestInverseKinematics (Failed)
	 94 - UnitTestInverseKinematicsMatrixViewAndSpan (Failed)
	 95 - memcheck_UnitTestInverseKinematicsMatrixViewAndSpan (Failed)

[traversaro]

Probably we just convert the old noFixedJoint to onlyRevolute, this should simplify the implementation and cover the old behaviour.