Add v1alpha2 MPIJob controller

[terrytangyuan]

This is the new MPIJob controller based on the API design in #95. Still a work-in-progress but creating a PR here for any immediate feedback.

Note that I created a new folder named "controllers" that includes both v1alpha1 and v1alpha2 MPIJob controllers.

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This change is 

[gaocegege]

Feel free to @ me when the PR is ready to review.

[terrytangyuan]

This PR is now ready for review.

Note that this controller is not completely finished yet as some tests are still failing but I do not have dedicated time for this anytime soon. I see continuous contributions from the community to v1alpha1 controller but eventually require more work to land in v1alpha2 controller so I want to wrap up this PR quickly so new contributions could land in new version of the controller going forward (avoid long-running branch). The community is welcome to help fixing the tests and I will certainly look into more if I have time.

This is based on an older version ever since #95 so there are some changes that are not in this new controller yet (which could be some follow-up work by the community but keeping them here for reference later): #106 #105 #100.

/cc @gaocegege @rongou @madhukarkm @Nivedita-V

[rongou]

/lgtm

[gaocegege]

/approve

[terrytangyuan]

Thanks everyone!
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gaocegege, terrytangyuan

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