Update guide for PXE multi-architecture setup
Update guide for PXE multi-architecture setup

This updates documentation for PXE multi-architecture setup in
the install guide. It clarifies the values for the two *_arch
config options and combines the two examples into one.

Change-Id: I10684e16dd6efcc6d17fb4a825e55286c16a20be
Related-Bug: #1582964

openstack/ironic,SauloAislan/ironic,pshchelo/ironic,SauloAislan/ironic,pshchelo/ironic,openstack/ironic,NaohiroTamura/ironic,NaohiroTamura/ironic