Allow chef-solo to run as non-root user

[tbe]

Chef allows to be run as non-root user [1].

But currently, dokken does not allow such a scenario for testing.

The sandbox directories are created with mode 0755
https://github.com/someara/kitchen-dokken/blob/e154ab66c3a375d6d82fa970d3c41c0f77e0a601/lib/kitchen/helpers.rb#L124

If the user within the container is not root and has a different uid then the user on the host, chef-client run fails with:

[2018-04-20T13:30:58+00:00] FATAL: Errno::EACCES: Permission denied @ rb_sysopen - /opt/kitchen/cache/chef-client-running.pid
[2018-04-20T13:30:58+00:00] FATAL: Errno::EACCES: Permission denied @ rb_sysopen - /opt/kitchen/cache/chef-client-running.pid

It would be great if mode for this directories could be changed from the kitchen configuration, to allow such scenarios.

[1] https://docs.chef.io/ctl_chef_client.html#run-as-non-root-user