use ext4 for guest default ephemeral

this isn't upstream default because of compatibility questions
with really old host on providers. However there is no reason not
to do it in devstack.

Change-Id: I6438c0efb297cfa5d3dbb5f00701b24f01c39d14
Merge "use ext4 for guest default ephemeral"
Merge branch 'master' of github.com:openstack-dev/devstack