Mount ext3 filesystems as ext4 on single use slaves

It has been suggested that tests may be happier with ext4 instead of
ext3 on the test slaves. At the end of building an image for single use
slaves modify fstab so that when slaves are booted with the image they
mount / as ext4.

Related-Bug: 1273386
Change-Id: I88409e734b1599a5517abfd9d76c7fd622c01bb5
Merge "Mount ext3 filesystems as ext4 on single use slaves"