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 variables, before generate_sample.sh
inspect the code to always have the default value.

Change-Id: I4f41eece3d93633fcc07274f533adfa1887325fc