spec: tweak pod definition
spec: tweak pod definition

- remove mount namespace requirement and instead switch to "shared
  volumes" language. As raised previously elsewhere [1][1] [2][2], this
  is somewhat moot since applications start chrooted into their
  respective filesystems anyway.
- Add illustrative explanation of the significance of the namespaces

[1]: https://github.com/GoogleCloudPlatform/kubernetes/pull/4701#discussion_r25125656
[2]: https://github.com/appc/spec/issues/318

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