Add set-pip-vars script to simplify pip manifests

Adding a scripts/set-pip-vars script that simplifies the setting
of DIB_PIP_MANIFEST_ environment variables from pip manifests for
disk image builds

Intent is that developers can run e.g. :

 source <(set-pip-vars ${TRIPLEO_ROOT}/undercloud_manifest)

To set all their DIB_PIP_MANIFEST environment variables for
building that stack of tripleo.

e.g. deploy-ramdisk, seed, undercloud, overcloud-control, overcloud-compute

In addition that they can use it to alter their setup:

e.g. source <(set-pip-vars my-manifest/dib-pip-manifest*client)

Change-Id: Ibdd9a0e928bcea87f024a8e980bcc816ff1fd0a6
Merge "Add set-pip-vars script to simplify pip manifests"