Update sample config generator script

This patch reverts 13d80020. The help message should be added
in oslo-incubator since generate_sample.sh is synced from there also.

Resolves bug #1204204

9f2b93b Sample config file generator clean up
33f9f78 Replaces relative path to absolute path in conf file generator
c97e17b Refactors if statement in config generator
8b11fda Provide useful defaults for config generator
4c02e0a Unset OS_xx variable before generate configuration

Change-Id: I96fcb39c0c620c147fea5c96416e506a758619ce
Merge "Update sample config generator script"