added config option 'chef_path' for chef_solo provisioner to be independ...

I had the problem of being forced to use company AMIs for the automation of our chef-server cookbook testbuilds. These AMIs were built with a preinstalled chef 10 into /usr/local/bin. Unfortunately the images cannot be upgraded to chef 11 as there is a vast number of legacy chef-solo scripts that are used by another departement and migration is not an option atm for them. 

I Added the config option 'chef_path' for chef_solo provisioner to be independent of target systems $PATH settings. If 2 different chef versions are installed you can now point testkitchen to the binary directory you actually want to use. E.g. chef_path: '/opt/chef/bin' to use omnibus chef-solo instead of the one in /usr/local/bin

Perhaps this can be helpful to others too.