webui: Use sections to group options belonging together

Also change order of options, so it all feels a bit more natural.
Merge pull request #121 from vuntz/use-sections

webui: Use sections to group options belonging together
Allow to select rabbit as notifier_strategy

RabbitMQ reporting is needed for Ceilometer integration.
webui: Only display file/swift options when relevant backend is used
webui: Improve strings
Merge pull request #123 from dirkmueller/release/pebbles/master

Allow to select rabbit as notifier_strategy
Merge pull request #125 from vuntz/improve-webui

webui: Only display file/swift options when relevant backend is used