Tidy up configuration sections and options

[nevali]

It's very poorly-documented, but the [twine] section actually specifies defaults used by the programs and modules.

To make this clearer, we should:—

1. Rename the [twine] section to [defaults]
2. Rename [twinecli] to [cli]
3. Warn on any occurrence of the deprecated sections or options[log] section
4. Provide a properly-documented up-to-date configuration file example

Tracked as RESDATA-1273

[nevali]

Requires bbcarchdev/libcluster#23