Move default_instance_type into nova.compute

Apart from the tests, this option is only used in
nova.compute.instance_types so move it there.

blueprint: scope-config-opts
Change-Id: Iadf4011739e50455b7ad4214a778a02e244fef35
Move common virt driver options to virt.driver

The default_ephemeral_format and use_cow_images options are used by a
number of virt drivers and nowhere else.

blueprint: scope-config-opts
Change-Id: I4ab2891e156589ae7bb44e6ce69cba3680a070c5