Planex 0.21.0

• Add planex-create-mock-config, which constructs a mock configuration
  file from the system yum configuration.
• planex-make-srpm: Add Provides: tags containing the Git hashes of the
  sources used to build binary RPMs, if this information is available.
• Makfile.rules, planex-patchqueue: Always rebuild pinned patchqueue tarballs,
  but short-circuit the rebuilding of dependent source and binary RPMs if the
  contents of the patchqueue have not changed
• planex-clone: Continue if cloning one of a series of pinned repos fails
• Makefile.rules: add a dependency on the mock config