puppet/apply.sh: provide fallback for EC2 IP retrieval

Debian's AMI[1] doesn't provide anything
inside 'facter ec2_public_ipv4', so let's try
the manual approach then before really falling
back to 'ip addr ...' hackery to avoid retrieval
of internal IP on Debian's AMI/EC2 system.

[1] http://wiki.debian.org/Cloud/AmazonEC2Image