Trigger cfn-init via script rather than lib

The current cfn-init software-config hook doesn't work at all
since cfn_helper is installed in a venv, not in site-packages.

This change switches to writing out metadata to
/var/cache/heat-cfntools/last_metadata and invoking cfn-init.

/var/lib/heat-cfntools/cfn-init-data cannot be used since that is
already populated with all boot metadata, which may still be used
by os-collect-config bootstrapping.

Change-Id: I7252a6f12223613b55b4b6417383673faa0d52b3
Closes-Bug: #1321513
Merge "Trigger cfn-init via script rather than lib"