Fix unhandled exception when trying to reference koji_tags

Fixes this traceback:

Releasing to target: test-el6-x86_64
Working in: /tmp/tito/release-adagios-ffd9f2b328fb0a2226042b0b3ce1a5c4e4d95af4TRmZQM
Traceback (most recent call last):
  File "/usr/bin/tito", line 23, in <module>
    CLI().main(sys.argv[1:])
  File "/usr/lib/python2.7/site-packages/tito/cli.py", line 94, in main
    return module.main(argv)
  File "/usr/lib/python2.7/site-packages/tito/cli.py", line 634, in main
    auto_accept=self.options.auto_accept)
  File "/usr/lib/python2.7/site-packages/tito/release.py", line 399, in __init__
    prefix="yumrepo-")
  File "/usr/lib/python2.7/site-packages/tito/release.py", line 292, in __init__
    if self.releaser_config.config.has_option(koji_tag, "scl"):
AttributeError: ConfigParser instance has no attribute 'config'