appc/spec: Collapse app and fileset manifests
appc/spec: Collapse app and fileset manifests

This reworks the app image and fileset concepts to collapse them into a
single manifest, the App Image Manifest. The execution details are now
optional. It also describes a dependency retrieval/matching process
based on labels.

mesosphere/spec,apcera/appc-spec,krnowak/appc-spec,marineam/spec,dgonyeo/spec,mesosphere/spec,dustymabe/spec,krnowak/appc-spec,krnowak/appc-spec,dustymabe/spec,apcera/appc-spec,zenlinTechnofreak/spec,sgotti/spec,ChengTiesheng/spec,philips/spec,3ofcoins/appc-spec,marineam/spec,zenlinTechnofreak/spec,endocode/appc-spec,squeed/spec,craimbert/spec,appc/spec,stufflebear/spec,craimbert/spec,jonboulle/spec,dustymabe/spec,iaguis/appc-spec,appc/spec,ChengTiesheng/spec,philips/spec,endocode/appc-spec,alban/appc-spec,3ofcoins/appc-spec,marineam/spec,iaguis/appc-spec,alban/appc-spec,squeed/spec,zenlinTechnofreak/spec,dgonyeo/spec,stufflebear/spec,stufflebear/spec,ChengTiesheng/spec,tklauser/spec,yifan-gu/spec,jonboulle/spec,tklauser/spec,tklauser/spec,apcera/appc-spec,squeed/spec,yifan-gu/spec,sgotti/spec,yifan-gu/spec,appc/spec,alban/appc-spec,3ofcoins/appc-spec,craimbert/spec,endocode/appc-spec,dgonyeo/spec,jonboulle/spec,iaguis/appc-spec