Changed 'exclusive' to default to 'false' instead of 'true' to prevent a...

Changed `$exclusive` to default to `false` instead of `true` to prevent accidentally nuking all default sysctl values as provided by the distribution.

As I wasn't sure, what your idea behind using `$exclusive = true` as default was, I created this pull request to allow a discussion about this.

I think in the most cases it might make sense to keep the sysctl defaults and if someone decides to use Puppet exclusively for his sysctl configuration, he might still set `exclusive = true`.

Feel free to dismiss this PR, I just wanted to have a place to initiate a discussion about this.