Allow .gitkeep in the jsonfile fact cache dir

##### Issue Type:

“Feature Pull Request”

##### Ansible Version:

devel

##### Environment:

Ubuntu 12.04 managed from Arch

##### Summary:

I would like to add a `.gitkeep` file to the `cache_dir` used by the `jsonfile` fact cache so I can run ansible from a fresh checkout of my repo without any additional setup. This PR modifies the `jsonfile` fact cache so that it ignores hidden files in the `cache_dir`.

RFC @bcoca