Make Ironic tests stop using file injection

Ironic does not support file injection, but our devstack-gate tests
are currently forcing Nova to test libvirt with file injection,
leading to many failing tests with libguestfs.

Related-bug: #1275301
Change-Id: I50967f02ce9c05174e1b4ceb162fc066357bb8f0
Merge "Make Ironic tests stop using file injection"