A no-op deploy driver for PXE boot

Towards the goal of "diskless" boot, but different than boot-from-volume, this blueprint proposes to create in a new deploy driver following the code pattern of the iscsi_deploy driver. However, instead of using the deploy ramdisk to for facilitating the iSCSI image transfer while the conductor is waiting for a call-back, the no-op driver will simply boot the node directly to the pxe target "boot", skipping "deploy". In effect, it will skip the deploy ramdisk boot cycle and will not automatically populate the "root=" field in the pxe config. In order to have a useful deployment, the feature "Override PXE options via Glance property" [1] is used to specify the root device in the glance image properties or the node's instance_info. t will then be up to the operator to configure the method of boot that they specify in the PXE cmdline. For example: kernel_cmdline="root=nfs:10.0.1.1:/images/my-nfsroot-image". In this case, the operator would be responsible for configuring a NFS server at 10.0.1.1 with an appropriately configured nfsroot image.

[1] https://review.openstack.org/#/c/177859/