provider/virtualbox: fix NFS tests on Windows
core: Respect VAGRANT_HOME everywhere

Use `VAGRANT_HOME`env var also for `Plugin::Manager` etc., not only in
the `Environment` class.
provisioners/docker: fix wrong variable usage

caught by @pda
Merge pull request #2837 from tmatilai/vagrant_home

core: Respect VAGRANT_HOME everywhere
core: tests for shared_helpers.rb

/cc @tmatilai
core: tests for more shared_helpers
synced_folders/nfs: Register `nfs_client_install` caps to correct distros