Change default ephemeral FS to ext4.

The ext3 filesystem does orders of magnitude more I/O than ext4 in
mkfs - a brief benchmark showed 14m to make a 1TB ext3 filesystem
vs 7 seconds to make a 1TB ext4 filesystem, with the ext3 filesystem
doing upwards of 6GB I/O.

Since ext4 has been a default in all major Linux distributions for
a long time now - RHEL5 has it as default, for instance - there is
no reason to default to ext3: users that need ext3 can configure
that themselves.

DocImpact

Change-Id: I833356af81df743a63d4442d389be82bc25f3914
Merge "Change default ephemeral FS to ext4."