Restrict Image Types

Today all image types are allowed to boot if an image of that type is present
for a flavor in Glance. In some cases, the operator may want to restrict a
flavor from being able to boot a certain image type.

Examples of this are:

A diskless flavor offering might have limited space on the hypervisor. In this
case, you would want to avoid using space on the disk by either booting from
volume or utilizing use a small iPXE boot ISO to boot the operating system over
the network. In this case, we'd restrict all image_types except for ISO.

In another case, you might want to restrict QCOW images from being built on a
XenServer environment. You'd want to ensure that QCOW and any other types of
images are restricted, while allowing VHDs.