support old configs without num_threads option set

If we migrate from spacewalk-splice-tool <= 0.25 we'll get following exception:

Traceback (most recent call last):
  File "/usr/bin/spacewalk-splice-checkin", line 93, in <module>
    checkin.main(opts)
  File "/usr/lib/python2.6/site-packages/spacewalk_splice_tool/checkin.py", line 186, in main
    spacewalk_sync(options)
  File "/usr/lib/python2.6/site-packages/spacewalk_splice_tool/checkin.py", line 114, in spacewalk_sync
    num_threads=CONFIG.getint('main', 'num_threads'))
  File "/usr/lib64/python2.6/ConfigParser.py", line 340, in getint
    return self._get(section, int, option)
  File "/usr/lib64/python2.6/ConfigParser.py", line 337, in _get
    return conv(self.get(section, option))
  File "/usr/lib64/python2.6/ConfigParser.py", line 541, in get
    raise NoOptionError(option, section)
ConfigParser.NoOptionError: No option 'num_threads' in section: 'main'

my patch provides default value (1) to solve this issue.