feature: support configuration of inject_partition with libvirt-image-backend=rbd

[Nobuto Murata]

At this moment, inject_partition=-2(disable) will be set when libvirt-image-backend=rbd is used. But in some specific cases, e.g an instance for NFV usecase does not have cloud-init, file injection is still needed.

I understand that it's not a recommended way to setup instances, but it would be nice if inject_partition is configurable.

https://github.com/openstack-charmers/charm-nova-compute/blob/master/templates/mitaka/nova.conf#L166-L173
{% if libvirt_images_type and rbd_pool -%}
images_type = {{ libvirt_images_type }}
images_rbd_pool = {{ rbd_pool }}
images_rbd_ceph_conf = {{ libvirt_rbd_images_ceph_conf }}
inject_password = false
inject_key = false
inject_partition = -2
{% endif -%}