Unset OS_xx variable before generate configuration

When generate_sample.sh generate the configuration of the
keystone middleware part, it use OS_XXXXX environment variable
if this one is set instead of the default one.

This change unset this vairables, before generate_sample.sh
inspect the code to always have the default value.

Change-Id: I365387e99dff0e1c64cd1c4198088200bf726cd7