Enable available_apps api call to take optional params

[Michael Nelson]

Currently the SCA client api call 'available_apps' requires lang, series and arch (as the SC client is only interested in the apps for a specific series/arch of the host machine).

But other users of the API (now the Ratings and Reviews server) would benefit from being able to request all the available_apps, irrespective of which distroseries/arch/language (and caching the result, rather than calling the api for each different distroseries, arch).

Note: currently the result for each app includes a 'series' key with a dict of series/arches for which the app is available.

[Anthony Lenton (elachuni)]

sca's API now accepts "any" for either of the lang, series or arch args, in which case it'll return all available langs, series or archs respectively.

I've filed bug #856808 in rnr-server to start using this feature.