fix: Respect metadata supplied for GCE builders.

Previously, any per instance metadata supplied via the GCE builder
was ignored.

Test plan:

- make test
- Manual testing via:
-- Created a packer config that contained a GCE builder with custom
   metadata set.
-- Ran `packer build`.
-- Verified the instance had the correct metadata in the GCE console.
Merge pull request #1563 from mpage/fix-gce-metadata

fix: Respect metadata supplied for GCE builders.