iPXE to use Swift Temporary URLs

This blueprint adds support for generating Swift temporary URLs for the
deploy and image's ramdisk(s) and kernel(s) when booting with iPXE.

Currently the iPXE driver requires an external HTTP server to serve
the deploy and image's ramdisk and kernel. When used with Glance, the
``ironic-conductor`` fetches the images from it and place them under the
HTTP root directory, and if a rebalance happens in the hash right the
new ``ironic-conductor`` taking over the node have to do the same thing,
fetch the images and cache it locally to be able to manage that node.

Having an external HTTP server should not be required when Glance is used
with a Swift backend, with Swift we can generate temporary URLs that can
be passed to iPXE to download the images without requiring credentials.