consolidate sources into common.json

As a first step, we will pull the common sources into common.json, maybe later into a sources.json

so instead of having in provider_base/common.json and in provider_base/provider.json an 'apt' resource, instead that gets put under a 'sources' hierarchy, and instead of having in webapp.json where the webapp source comes from, and something for nickserver, tapicero, leap-mx, etc. everything gets pulled under the 'sources' hierarchy in the common.json

https://github.com/leapcode/leap_platform/pull/61

(from redmine: created on 2015-02-05, closed on 2015-02-05, blocks #6697 (closed))