Create pip manifest files of installed packages

Create the pip-manifest element to encapsulate the saving and use
of pip manifest files.

Ensure that it installs prior to any elements that should be able
to use it.

Write pip manifest files for installs in:
    * os-svc-install
    * os-apply-config
    * os-collect-config
    * os-refresh-config
    * openstack-clients

Enable using a manifest file to reinstall exact versions of
packages in a subsequent run through the DIB_PIP_MANIFEST_*
environment variables.

Change-Id: I4d4ab5921c534727b48cb9969ec8ecfd2c26e6ed
Merge "Create pip manifest files of installed packages"