(MODULES-8823) Add support for overlay option in zfs

This option allows mounting over non empty directories. The current
master of zfs_core simply doesn't implement such option on the zfs
resource. This change allows for specification of such option in
puppet modules:

zfs { 'my_pool/my_volume':

ensure => present,
mountpoint => '/mnt/non_empty_dir',
overlay => 'on'

}