Add nodes/.gitignore to gitignore.

This allows the nodes/* to be placed under its own git-repo, so users
can manage the nodes/ directory in a separate git repo.
For that, the users probably want a .gitignore file in there too, to
ignore e.g. the sample_host.json.
Merge pull request #140 from berkes/feature/nodes-in-repo

Add nodes/.gitignore to gitignore.