Various fixes, including
*  Split up nscd executes due to Chef 11.6.0 InvalidResourceSpecification error
* Simplify .kitchen.yml
* remove centos tests (see COOK-1987)
*  remove ubuntu-1004 tests (error seems related to
      https://bugs.launchpad.net/ubuntu/+source/openldap/+bug/690815)
*  add pessimistic locks to Gemfile & Berksfile
*  use hostsfile cookbook inside openldap-test instead of host