Adding MPI python sdk

[johnugeorge]

MPI python sdk is added with e2e test.

This PR has extra changes

1. job-role for MPIReplicaTypeLauncher pod is set to master in MPI controller so that logs are fetched from launcher pod by default. This is consistent with other frameworks.
2. In sdk, training.kubeflow.org prefix is added to job-role, job-name and replica-index labels which are used for getting pod names.

[coveralls]

Pull Request Test Coverage Report for Build 2472869982

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• 8 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.1%) to 37.034%

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Files with Coverage Reduction	New Missed Lines	%
pkg/controller.v1/tensorflow/tfjob_controller.go	3	70.62%
pkg/controller.v1/mpi/mpijob_controller.go	5	78.37%

Totals
Change from base Build 2471254094:	0.1%
Covered Lines:	2315
Relevant Lines:	6251

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💛 - Coveralls

[terrytangyuan]

Thanks!

/lgtm

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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

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