stack.sh: Clear OpenStack related envvars
stack.sh: Clear OpenStack related envvars

stack.sh usually fails when it is started in a shell session
where OpenStack related environment variables OS_* are set.
Most common failure scenarios are failures in keystone operations.

This commits clears OpenStack related environment variables
at the begining of stack.sh.

Change-Id: I3a924a0586dc9bb28f3bf3e151e100c24015efe5

dtroyer/devstack,openstack-dev/devstack,dtroyer/devstack,nawawi/openstack,nawawi/openstack,openstack-dev/devstack