Support hypervisor_type option

Currently, in order to deploy an instance to specific hypervisor, user needs to create an instance using a glance image which has hypervisor_type metadata. However using glance metadata, admin have to prepare images as many as hypervisors with setting hypervisor_type metadata previously.

When we use hypervisor specific image types such as qcow2, vodka, vhd, it is acceptable to prepare images as many as hypervisors, however if we use hypervisor non-specific image type such as raw format, same image should be available for KVM, Baremetal, Xen, LXC etc hypervisors. But using current glance metadata mechanism, we have to prepare image store region for each hypervisor even if we can share same image between multiple hypervisors.

In order to solve the situation,

 * Nova supports hypervisor_type key which is defined in the extra-spec of flavor.
 * Nova supports hypervisor_type as an rgument for boot option of server API.