Fixes bugs related to handling of boolean config values

* Changed direct casts to bool to a boolean comparison with
  string values. A direct boolean cast of a string in Python
  will not work (any non-empty string returns True, empty string
  returns False)
* Modified get_boolean of the BaseConfigSectionInterface to return
  the override value if it exists, rather than the value of the
  override or'ed with the configured value

Change-Id: Ib6bb142e170d531ef5a176a2f2a3a03e3cce0907
Merge "Fixes bugs related to handling of boolean config values"