Review Request 52135: Implemented the NESTED_CONTAINER_* calls in the agent API.

[Benjamin Mahler]

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Review request for mesos and Vinod Kone.


Bugs: MESOS-2449
    https://issues.apache.org/jira/browse/MESOS-2449


Repository: mesos


Description
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This patch adds the wiring for the NESTED_CONTAINER_* calls,
including validation and calling into the containerizer.


Diffs
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  src/slave/http.cpp e1ba653b222189ab63f303ff6295becec25a5919 
  src/tests/api_tests.cpp 26f99f7c337fbbc5278d1b30d3d5c8f659ddf5ca 

Diff: https://reviews.apache.org/r/52135/diff/


Testing
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Added a test that mocks out the containerizer.


Thanks,

Benjamin Mahler