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.