Added DC/OS integration tests for UCR.
Added DC/OS integration tests for UCR.

This patch included new DC/OS UCR integration test for app and pods:
1) Tested image default entrypoint.
2) Tested scratch image support.
3) Tested image with whiteout files.
4) Tested image by digest.

BenWhitehead/dcos,lingmann/dcos,mellenburg/dcos,BenWhitehead/dcos,surdy/dcos,branden/dcos,surdy/dcos,branden/dcos,kensipe/dcos,lingmann/dcos,mnaboka/dcos,mellenburg/dcos,surdy/dcos,mesosphere-mergebot/mergebot-test-dcos,mesosphere-mergebot/dcos,mellenburg/dcos,mnaboka/dcos,lingmann/dcos,jeid64/dcos,mesosphere-mergebot/dcos,darkonie/dcos,vishnu2kmohan/dcos,dcos/dcos,vishnu2kmohan/dcos,asridharan/dcos,jeid64/dcos,amitaekbote/dcos,asridharan/dcos,jeid64/dcos,mesosphere-mergebot/mergebot-test-dcos,amitaekbote/dcos,lingmann/dcos,darkonie/dcos,GoelDeepak/dcos,kensipe/dcos,jeid64/dcos,mnaboka/dcos,vishnu2kmohan/dcos,asridharan/dcos,mesosphere-mergebot/dcos,mnaboka/dcos,mnaboka/dcos,darkonie/dcos,vishnu2kmohan/dcos,mesosphere-mergebot/dcos,kensipe/dcos,amitaekbote/dcos,surdy/dcos,asridharan/dcos,mesosphere-mergebot/mergebot-test-dcos,mesosphere-mergebot/mergebot-test-dcos,BenWhitehead/dcos,dcos/dcos,amitaekbote/dcos,branden/dcos,GoelDeepak/dcos,dcos/dcos,branden/dcos,BenWhitehead/dcos,GoelDeepak/dcos,mellenburg/dcos,GoelDeepak/dcos,kensipe/dcos,darkonie/dcos,darkonie/dcos,dcos/dcos,dcos/dcos