Rework autoscaling group current_size attribute

The current implementation requires a path which is not needed.
rsrc.FnGetAtt('current_size', 'name')
it should allow:
rsrc.FnGetAtt('current_size')

Also move the tests to the correct test module and make them more
into unit tests.

Change-Id: Id05a1f09ecc6741cbf677baf52fbb58d93028ac4
Merge "Rework autoscaling group current_size attribute"