Replace DIB installation step from git clone to pip
Replace DIB installation step from git clone to pip

This replaces DIB installation step from git clone to pip
in Bare Metal Service Installation Guide so that all of DIB required
packages such as dib-utils can be installed automatically.
Without this change, diskimage-builder command fails due to
"dib-run-parts: command not found".
The install-dependencies script which tripleo-incubator provides
doesn't install all of DIB required packages.

Closes-Bug: #1414510
Change-Id: I0f68fc79b5addbf38713a42e8ac2d631e4f2f219

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