Revert change of default FS from ext3 to etx4

Change 2c3bff93269d9d7264e451cb17935313a56a1798  changed the default FS for
ephemeral disks from ext3 to ext4, but this has two problems:

i)  Not all GuestOSs support ext3, so making this change requires a notification period
so that existing images and configurations on running systems can be modified to
still use ext3

ii) Ephemeral backing files use default in their file name as a caching key.  This change
makes  "default"'s meaning is mutable and therefore the naming convention needs to
changed as part of the change to ext4 becoming the default FS.  Without this there will
be inconsistent behaviour between systems depending on whether the backing file
exists or not.

This change reverts the default to ext3 until these issues can be addressed.

Systems that need ext4 can achieve this use the existing configuration options.

Change-Id: I8de225729bbc77b263fcac2fa72b0e9cc6c1b2bf
Merge "Revert change of default FS from ext3 to etx4"