Add tftp mapfile configuration in install-guide
Add tftp mapfile configuration in install-guide

The document http://docs.openstack.org/developer/ironic/deploy/install-guide.html
just show how to create a tftp map file, but this have to be enabled in tftp
configuration file, else, it is not working as well.

In this patch, add the tftp map file configuration steps.
Closes-Bug: #1427953

Change-Id: I3d838580cbd60b3264b9378a547cadbae2136d36

naototty/vagrant-lxc-ironic,bacaldwell/ironic,NaohiroTamura/ironic,ionutbalutoiu/ironic,SauloAislan/ironic,redhat-openstack/ironic,devananda/ironic,naterh/ironic,bacaldwell/ironic,ionutbalutoiu/ironic,supermari0/ironic,dims/ironic,Tan0/ironic,pshchelo/ironic,openstack/ironic,NaohiroTamura/ironic,SauloAislan/ironic,rdo-management/ironic,pshchelo/ironic,openstack/ironic,naototty/vagrant-lxc-ironic,hpproliant/ironic,dims/ironic