Adding default value to untar command_options

in puppet 2.7 you cannot pass undef as a value, setting default value to undef
to avoid puppet error:

Must pass command_options to
Deploy::Untar[/tmp/deploy/atlassian-stash-2.10.1.tar.gz] at
/tmp/vagrant-puppet-2/modules-1/deploy/manifests/untar.pp:45 on node stash1