Morden bare metals usually support PXEs from all NICs, in
our current logic only the port matching mac address in the
BOOTIF will be set to pxe-enabled. This does not work for UEFI
nor user friendly to bonding.
This patch adds a configuration option [processing]update_pxe_enabled
to control whether this field should be updated according to
introspection data, defaults to False to keep backwards compatibility.
Change-Id: I6f3b00180f62dc6f500ac2cdb5d8f8cc7c7190cf