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

doc/source/deploy/install-guide.rst

@@ -444,16 +444,15 @@ them to Glance service:
 
 .. _script: https://github.com/openstack/tripleo-incubator/blob/master/scripts/install-dependencies
 
-   - Clone the project and run the subsequent commands from the project
-     directory::
+   - Install diskimage-builder package (use virtualenv, if you don't
+     want to install anything globally)::
 
-       git clone https://github.com/openstack/diskimage-builder.git
-       cd diskimage-builder
+       sudo pip install diskimage-builder
 
    - Build the image your users will run (Ubuntu image has been taken as
      an example)::
 
-       bin/disk-image-create ubuntu baremetal -o my-image
+       disk-image-create ubuntu baremetal -o my-image
 
      The above command creates *my-image.qcow2*, *my-image.vmlinuz* and
      *my-image.initrd* files. If you want to use Fedora image, replace
@@ -464,7 +463,7 @@ them to Glance service:
 
    - Build the deploy image::
 
-       bin/ramdisk-image-create ubuntu deploy-ironic \
+       ramdisk-image-create ubuntu deploy-ironic \
        -o my-deploy-ramdisk
 
      The above command creates *my-deploy-ramdisk.kernel* and