Support flavor with no local disks

Feature: Add no local_disks option to flavors

We need the possibilty to add a flavor with no local disks.
Which will result in no ephemeral (root) disk(s) on the hypervisor.

Currently, when creating a flavor with root disk size 0 the fallback is the size of the image, that gets created as block_device on the hypervisor. Therefore a new boolean parameter 'local_disks' (default to True) will be added to support no (root) disks locally on the hypervisor.

Background:

We do not want to have ephemeral disks locally on the hypervisor; not on a local filesystem nor on a share mounted to /var/lib/nova/instances.
Because, as that shares grows, they get unmaintainable and performance is getting worse.
On a local hypervsisor filesystem is also not always a good idea, because there are consumers, that do have data in the root disk as well, which sometimes cannot be simply thrown away.
For best flexibility and manageability we ideally want to have all disks as volumes.

We want to create a flavor, were we set local_disks=False. This flavor prevents consumers from creating instances with HV local block-devices. Users are then somehow forced to previously create a boot volume or to add the additional
block-device paramter with dest_type 'volume' upon their 'nova boot' request, to have a volume created.

For some customers we explicitly want to forbid using ephemeral storage at all.
In these environments (domains and/or projects) only flavors with local_disks=False will exist.

This change will add support for 'local_disk' to nova flavors.
Booting instances with that flavor, without providing a bootable volume, will fail early.