Add search_solo() that can return defaults

We're seeing a frequent pattern in recipes where a search is guarded with a check for chef solo, and also if no results are returned, feeding in a default value. This commit adds a search that is aware of chef-solo, and returns a default value if chef-solo is being used.

I know this probably isn't release-worthy, and may not work at all. Can you give me some suggestions for the best way to test these kinds of changes? I'm not sure chefspec will be running chef-solo, so I'm not sure a unit test like the others will work.

Thank you!