Only add RHEL specific options to the answer file when run on RHEL

While reversing parts of the previous patch on this topic since
they seemed to be too granular, this will make sure RHEL specific
questions are never asked on other linux distributions.

Change-Id: I8790c5a8cf0d97ba2a370c24584b8120aa8a9963
Cleaned packstack.installer.common_utils

- cleaned from useless functions
- refactored to be pep8 compliant
- separated to standalone modules
- added unit tests

Change-Id: If5a300fe04efab9e13549f2b4d05c1b28e2e40c6
Merge "Only add RHEL specific options to the answer file when run on RHEL"