Update 10gen_repo.rb

When attempting to add mongo via the 10gen repos, gpg check must be disabled!

In the `/etc/yum.repos.d/10gen.repo` file a `gpgcheck=0` directive must be added; this is not the default `yum_repository` behavior. I was unable to get the `mongodb::default` recipe to install mongo until I manually changed the `10gen.repo` file to read as

```
[10gen]
name=10gen Repository
baseurl=http://downloads-distro.mongodb.org/repo/redhat/os/x86_64
gpgcheck=0
```