Add note to pxe configuration doc
Add note to pxe configuration doc

Adding a note into the PXE setup instructions
indicating the existence of the ipxe boot interface
and that the ipxe_enabled option has been deprecated.

Change-Id: I4c553b7d5da080103f1d1ccf2573586d61e7cfc4
Story: #1628069

openstack/ironic,openstack/ironic