[COOK-4263] -  OpenVPN users recipe should run in Chef-Solo when chef-solo-search installed

I've added an additional check in the users recipe to see if the [chef-solo-search](https://github.com/edelight/chef-solo-search) cookbook is installed when running in Chef-Solo. If it is then data_bag search will work and the recipe can proceed normally.

The code that I used is taken from/inspired by the way that the [opscode users](https://github.com/opscode-cookbooks/users) cookbook detects if chef-solo-search is installed.

I'm not sure how to write tests for this, but for what it's worth I've used it on an Ubuntu 13.04 x64 Vagrant box and it works as expected. I have the repo set up [here](https://github.com/charlesbjohnson/vps_on_vpn).

Please see issue [4263](https://tickets.opscode.com/browse/COOK-4263) for more info.

Thanks.