3.2.0a4: dangling commas in service lists

[matthewwall]

1. clean install
2. check weewx.conf: no comma at end of archive_services
3. wee_extension --install forecast.tgz
4. wee_extension --uninstall forecast
5. archive_services now has a dangling comma

also, data_services starts out with a dangling comma. the code should be smart enough to deal with dangling commas, but we should not put them in weewx.conf in the first place and whatever wee_* utilities process weewx.conf should not leave them.

[tkeffer]

A dangling comma is actually ConfigObj's way of signalling that the contents are a list. To work around this, we developed utility weeutil.weeutil.option_as_list()