Only copy /etc/mtab over when it's not already there for default.tmpl [a=fginther] [r=daniel-thewatkins,philroche]
Otherwise, the build errors out when it tries to copy in the 'fake' mtab:
15:37:27 [ 23.982677] cloud-init[1366]: cp: '/etc/mtab' and '/tmp/builder.ne2f/etc/mtab' are the same file 15:37:27 [ 23.984259] cloud-init[1366]: Wed, 08 Aug 2018 15:37:18 +0000: Unable to copy mtab in place