Reduce code duplication in test_linux_dhcp
Reduce code duplication in test_linux_dhcp

This patch reduces duplication in test_output_opts_file_*()

Change-Id: Ie68b26a48c1d0ed9cb8e584a7284c06688937cf9
Closes-Bug: #1352870

gkotton/vmware-nsx,gkotton/vmware-nsx