nuageinit: tests: Cleanup

• Export NUAGE_FAKE_ROOTDIR only once
• Use the header section of the test to require the root user
• Use the PWD environment variable
• Set the root/sys shell as /bin/sh
• Use RFC 5737 reserved IP addresses

Signed-off-by: Jose Luis Duran <jlduran@gmail.com>
(cherry picked from commit e72457c4f5166eef2a27249e02f3c1e9a1cf852d)