Default ironic to not use nested KVM
Default ironic to not use nested KVM

Nested KVM has many benefits, but when it doesn't work it fails jobs.
This change defaults Ironic to using QEMU emulation, if someone wants
their local devstack to autodetect, they should set
IRONIC_VM_ENGINE="auto", or force KVM by setting
IRONIC_KVM_ENGINE="kvm".

Change-Id: I0367561ead972679250fc48d0f21b2c208e77e60

SauloAislan/ironic,openstack/ironic,NaohiroTamura/ironic,pshchelo/ironic,SauloAislan/ironic,NaohiroTamura/ironic,openstack/ironic,pshchelo/ironic