Revert "Support conditional functions"

This reverts commit 55bde6234b02bd63673e9ed22be44244069cb5a9.
Allow empty string to intrinsic function !GetAZs

As per spec !GetAZs "" is allowed and equivalent to !GetAZs $(AWS::Region)
Allow additional intrinsic conditional functions
Merge branch 'add-conditional-functions'
Merge branch 'allow-empty-getazs-value'