RpmSign: keyfile UX improvements

[ktdreyer]

• RpmSign: nicer logging when writing release.asc

  With the switch to using the DebRepo class, we were logging the DebRepo repr when we wrote the release.asc file.

  It's not terrible because the repr does show the path anyway, but it's less noise in the logs to just show the repository's filesystem path instead.

• RpmSign: allow relative paths for --keyfile

  We call os.chdir() a lot during signing. Prior to this change, we required the user to specify an absolute path to --keyfile.

  Do the absolute path translation ourselves to make this a bit more user-friendly.

[ktdreyer]

Fixes https://github.com/alfredodeza/merfi/issues/59

[ktdreyer]

@ahills @tserlin FYI