Read metadata from local heat-cfntools cache

Heat writes to /var/lib/heat-cfntools/cfn-init-data , and heat-cfntools
writes the last metadata it read to
/var/cache/heat-cfntools/last_metadata. We expect to run on first boot,
which is why we need the heat version. Then we also expect to run after
cfn-hup notices changes, so that is where the last_metadata comes from.

Change-Id: I781c74a10b419f84c6983ed2b4d54b47bfa7b605