Removing 'base_output' from ad-hoc actions specification

* Since '$' is a valid expression it may be used to refer to
  base action result in ad-hoc actions so using '$.base_output'
  is redundant.
* Fixed module expressions.py so that YAQL expression '$' is
  considered valid.
* Fixed corresponding test cases.

Change-Id: Ib7e534b3f9cc8fffb0f1626a987a8c8cc6eeddf6