Add --target options to create-tag and post-candidates release.py subcommands.

This makes it easier to use these commands on a different machine than where
the tarballs were built.

* tools/dist/release.py
  (sign_candidates, get_sha1info, get_siginfo): Factor common code into ...
  (get_target): ... this new function.
  (post_candidates, create_tag): Use get_target() to support target option.
  (main): Add --target to post-candidates and create-tag subcommands.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1486069 13f79535-47bb-0310-9956-ffa450edef68