Allow file_cache_path config for chef_solo

This enhancement would pave the way for `vagrant-cachier` to cache chef files in solo mode: fgrehm/vagrant-cachier#14

Currently, chef_solo's solo.rb template hardcodes `provisioning_path` as `file_cache_path` value:
https://github.com/mitchellh/vagrant/blob/v1.2.2/templates/provisioners/chef_solo/solo.erb#L4

chef_client's `client.rb` template allows it to be configured:
https://github.com/mitchellh/vagrant/blob/v1.2.2/templates/provisioners/chef_client/client.erb#L19

I would suggest that solo's `file_cache_path` default to `provisioning_path`, but have the option to set `file_cache_path` explicitly in the `Vagrantfile`.

PR forthcoming :)