961124: Allow rct dump-manifest to be called more than once

Added a --force/-f uption to dump-manifest. If this is used, it will overwrite existing files.
There is no ability to do this per file. The command either fails on the first file, or
overwrites them all.
Merge pull request #604 from candlepin/bkearney/961124

961124: Allow rct dump-manifest to be called more than once