Use webapp/nickserver:master on leap_platform:master

Use sources.platform.apt.basic:http://deb.leap.se/experimental-platform sources.webapp.revision:master sources.nickserver.revision:master

as default in leap_platform:master, and change those to the release branches when doing the release.

Todo:

• Upgrade the release docs
• Remove sources overrride on CI

(from redmine: created on 2016-12-06, closed on 2016-12-08)