Logging unification

Logging unification

Because of existing idea of tagging step in logs,
this commit introduces one more macro:

SCOPE_LOG_TAG(TAG)

Defining this macro in given scope will change log tag of that
scope.

Example:

class StepXYZ : public Step {
  void func() {
    LOG(DEBUG) << "Status is ...";
  }

  SCOPE_LOG_TAG(XYZ)
};

Every log from class StepXYZ will be place with "XYZ" tag.

Change-Id: I8a8e192b0a746dae19a961e238a798b9b2c0c4b9