Add missing python backend plugin dep on the old py commands.

Fixup listtargets test - use uniform address.spec.

Also do the same for filemap as a cleanup sweep on
  https://rbcommons.com/s/twitter/r/537/

Testing Done:
$ ./build-support/bin/ci.sh

Travis went green: https://travis-ci.org/pantsbuild/pants/builds/27698663

Bugs closed: 239

Reviewed at https://rbcommons.com/s/twitter/r/540/
Add support for multiple pants_support_baseurls.

Changed pants_support_baseurl to a list. Pants now goes through each url in the list from first to last, downloading the first binary with a valid url (and then breaking). This way, pants users can set their own support repo online and add it to the beginning of the list, so that their repo will be queried before the default repo on pantsbuild.github.io.