This patch is making the neutron DHCP provider to explicitly check if
the request is coming from iPXE before ACK'ing the PXE request with the
iPXE script (When iPXE is enabled only).
Also, update the documentation that explains how to configure neutron
for iPXE to create the "ipxe" tag based on the DHCP user class or DHCP
option 175 (as before).
Change-Id: I60f9f515a9bd45aed6b2f6f9165a2f85259b3879
Closes-Bug: #1454332
4ec47b6660