Make GCE_ACCOUNT_FILE_B64_BZ2 optional in packer-build-install
Make GCE_ACCOUNT_FILE_B64_BZ2 optional in packer-build-install

It isn't set during pull requests (since it's a secure variable) so
was previously causing the Travis run for PRs to fail, even though
it isn't actually needed for the tests to run.

Fixes #456.

travis-ci/packer-templates,travis-ci/packer-templates,travis-ci/packer-templates