Don't mix templates and cookbooks in same folder

Having YAML template definitions stored in the same base directory of a
set of cookbooks can be source of confusion, and does not follow
classical way to organize Chef cookbooks.

Note: we also should consider switching to Chef roles one day.
Update README.md

No longer relevant to refer to travis-images templates as they were
moved into travis-cookbooks repository.

[ci skip]
Update golang default attributes

- follow Chef guidelines
- defaults correspond to worker.standard.yml values

Note: we also could comment these attributes to force Chef to fail when
these attributes are not properly set, but that would breach usual Chef
styling.
Merge pull request #412 from travis-ci/gc-move-templates

Don't mix VM templates and Chef cookbooks in the same folder