Add relatime property support to zfs

[h0tw1r3]

need this myself, cleaned up #76 as author was unresponsive

[AriaXLi]

Hi @h0tw1r3 thanks again for cleaning this up and contributing! Since this attribute is only supported on Linux platforms, it's causing some issues in CI.

11:26:36   Error: Could not run: Execution of '/usr/sbin/zfs get -H -o value relatime tstpool/tstfs' returned 2: bad property list: invalid property 'relatime'
11:26:39   For more info, run: zfs help get

When you can, could you please update this PR to account for this?
We have some other Linux-only properties that handle this in type (for overlay):

puppetlabs-zfs_core/lib/puppet/type/zfs.rb

Lines 83 to 89 in e16b1c2

	newproperty(:overlay) do
	desc 'The overlay property. Valid values are `on`, `off`.'
	validate do |_value|
	raise Puppet::Error _('This property is only supported on Linux') unless Facter.value(:kernel) == 'Linux'
	end
	end

Or in provider (for aclmode):

puppetlabs-zfs_core/lib/puppet/provider/zfs/zfs.rb

Lines 60 to 69 in e16b1c2

	[:aclmode, :acltype, :shareiscsi, :overlay].each do |field|
	# The zfs commands use the property value '-' to indicate that the
	# property is not set. We make use of this value to indicate that the
	# property is not set since it is not available. Conversely, if these
	# properties are attempted to be unset, and resulted in an error, our
	# best bet is to catch the exception and continue.
	define_method(field) do
	zfs(:get, '-H', '-o', 'value', field, @resource[:name]).strip
	rescue
	PARAMETER_UNSET_OR_NOT_AVAILABLE