Include CPU Mode in image_meta for Glance

Add cpu_mode in image_meta through glance metadata defs. The CPU Mode values are: ``none, custom, host-model and host-passthrough``. For the common hypervisor options like kvm and qemu, the cpu-mode is set to ``host-model`` by default. The image_meta value will take secondary precedence after flavors to override thi...

Modify CPU Mode precedence for OpenStack Compute (nova)

The CPU mode is set to host-model now. This blueprint will check if any cpu mode is specified in the image_meta or flavors. If so, the flavor value will take the precedence over image_meta. If cpu mode is not defined in either locations, then the default value of host-model will be set