Add docker nodesets for CentOS 6 and 7
Ran a successfull test with:
```BEAKER_debug=yes PUPPET_INSTALL_TYPE=agent \ BEAKER_set=centos-6-x64_docker BEAKER_destroy=no \ bundle exec rspec spec/acceptance/puppetserver_latest_spec.rb```
Add docker nodesets for CentOS 6 and 7
Ran a successfull test with:
```
BEAKER_debug=yes PUPPET_INSTALL_TYPE=agent \
BEAKER_set=centos-6-x64_docker BEAKER_destroy=no \
bundle exec rspec spec/acceptance/puppetserver_latest_spec.rb
```