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
Move some AutoScalingGroup validation tests to a separate file

Change-Id: I56da94e89d206b4277c8ee03ebb23c3f20c08853
part of blueprint decouple-nested
Convert the autoscaling policy tests into unit tests

Change-Id: I97e13307231c61da7147e02f5706c86cda10c0d9
part of blueprint decouple-nested
Rewrite AutoScalingGroup adjustment tests as unit tests

Change-Id: I1c752243ef981a19ff1572071f43e5eaebe3b0dc
part of blueprint decouple-nested
Merge "Rework autoscaling group current_size attribute"
Merge "Move some AutoScalingGroup validation tests to a separate file"
Merge "Convert the autoscaling policy tests into unit tests"
Merge "Rewrite AutoScalingGroup adjustment tests as unit tests"