Enable callers outside of bosh-lite directory

I keep trying ot run make_manifest_spiff from the cf-release directory
and running into the following:

  ../bosh-lite/scripts/make_manifest_spiff
  cp: cannot stat `manifests/cf-stub-spiff.yml': No such file or directory

Using paths relative to the script helps resolve this inconvenience.
Merge pull request #62 from sykesm/execute-outside-boshlite

Enable callers outside of bosh-lite directory