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
e852f5f16c