Devstack: allow extra PXE params
Devstack: allow extra PXE params

This adds a variable IRONIC_EXTRA_PXE_PARAMS that can be used in
devstack configuration to add additional pxe parameters to the
pxe_append_params config in ironic.conf.

For instance, someone could use this to set 'noapic' in the kernel
command line for pxe boots.

Change-Id: I08d468defd3187f7837801c99ee625928cc92c47

devananda/ironic,pshchelo/ironic,bacaldwell/ironic,openstack/ironic,NaohiroTamura/ironic,SauloAislan/ironic,SauloAislan/ironic,pshchelo/ironic,openstack/ironic,NaohiroTamura/ironic,bacaldwell/ironic