iPXE dynamic configuration for Ironic

The current iPXE support depends on configuration files to be cached in the disk. This creates a dependency between a given ironic-conductor a given node (even without a conductor lock on a node) because that ironic-conductor is the only one able to boot that node. This also makes take-over more complicated because...

Allow a mechanism to override the PXE boot options for a unique image via glance properties for Ironic

Fedora Atomic 21 requires the following command line to boot properly: pxe_append_params = nofb nomodeset vga=normal console=tty1 no_timer_check rd.lvm.lv=atomicos/root root=/dev/mapper/atomicos-root ostree=/ostree/boot.0/fedora-atomic/a002a2c2e44240db614e09e82c7822322253bfcaad0226f3ff9befb9f96d315f/0 Overridding ...