stbt.conf: Add default configuration values for `stbt batch`
stbt.conf: Add default configuration values for `stbt batch`

Since the refactoring to Python in #495, `stbt batch run` was failing
with:

> ConfigurationError: No section: 'batch'

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester