core: only check for box existence if not force [GH-2757]
synced_folders/nfs: automatically install NFS client if possible

If guests have the following capabilities, automatic NFS client
installation will be done:

  * nfs_client_installed - Checks if the NFS client is installed
  * nfs_client_install - Install the NFS client

Support is already in for Debian, Ubuntu, RedHat, CentOS, and Fedora