Implement config.get_sections() to clarify how sections can be used.
* bzrlib/tests/test_config.py: (TestConfigGetSections): Expose the various supported section use-cases.
* bzrlib/config.py: (IniBasedConfig.get_sections, GlobalConfig.get_sections) (LocationConfig.get_sections, BranchConfig.get_sections): Properly capture the different section usages exposed by the various config objects.