Remove 'import nodes.pp' from manifests/site.pp

According to https://docs.puppetlabs.com/puppet/latest/reference/lang_import.html
the import keyword is deprecated, and the following in puppet.conf is sufficient:
manifest = $confdir/environments/$environment/manifests
Add default options for various providers in site.pp
Merge pull request #2 from tampakrap/sitepp

Remove 'import nodes.pp' (deprecated). Add default options in site.pp