Make runs reproducible when using --answer-file

Until now, using --answer-file could lead to small changes that
made runs non-reproducible. This patch aims at fixing that with
the following changes:

- Overwrite the answer file passed as a parameter with the
  processed answers. This is useful when enabling features that are
  not there by default, and end up with PW_PLACEHOLDER value in
  some variables.
- Make sure CONFIG_HEAT_AUTH_ENC_KEY is respected if entered in
  answer file.
- Make sure nova migration keys are not regenerated if already
  present.

Change-Id: I6837a2973aed9d29d6c44ac64b510f91b80f878a

packstack/installer/run_setup.py

@@ -639,6 +639,10 @@ def _main(options, configFile=None, logFile=None):
         path = _getanswerfilepath()
         if path:
             generateAnswerFile(path)
+    # If an answer file was provided, some options may have been overriden
+    # Overwrite answer file with updated options
+    else:
+        generateAnswerFile(options.answer_file)
 
     # Update masked_value_list with user input values
     _updateMaskedValueSet()