RPM Support

This is a request to provide a supported command line tool, installable as an RPM, to generate PULP_MANIFEST files in the right format for a directory structure. This should handle recursive generation as well as being able to re-run it and generate an updated PULP_MANIFEST.

There are a few examples of this in use today:

1. http://docs.pulpproject.org/plugins/pulp_rpm/tech-reference/iso-plugins.html#iso-importer
2. https://gist.github.com/jlsherrill/321cf092fce13d1c4f3a
3. https://gist.github.com/jlsherrill/e7c72e1ed82379955c2208ac472b0be7

Rather than have floating around tools, users would benefit from a simple, supported tool from the Pulp project that properly generates what Pulp requires to sync a repository.

Requirements for the tool:
- name = 'pulp-manifest'
- it takes 1 positional argument that specifies a path to the directory where the PULP_MANIFEST file should be created
- it creates PULP_MANIFEST file by walking the directory structure recursively and recording information about files in the CSV format: file_name,checksum,file size
- if a PULP_MANIFEST already exists, it should be regenerated
- pulp-rpm.spec file should be updated to have this tool installable as a separate RPM.