Expose main container name as a configurable field

[ywskycn]

Expose main container name as a configurable field. This is used for cases where launcher/worker pods have sidecars. Existing kubectl exec command in mpi_job_controller.go doesn't include container name, causing the command fails.

---

This change is 

[terrytangyuan]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: terrytangyuan

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [terrytangyuan]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[terrytangyuan]

@ywskycn Could you fix this?

ERRO Running error: buildssa: analysis skipped: errors in package: [/home/travis/gopath/src/github.com/kubeflow/mpi-operator/pkg/controllers/v1alpha2/mpi_job_controller_test.go:180:2: too few arguments in call to NewMPIJobController] 

[ywskycn]

@terrytangyuan , may need to change this PR. In the v1alpha2 spec, users can specify container name, thus a global config main_container may be not reasonable.

One approach would be, we read the worker's container name on-the-fly for each job, ant put the name in the kubectl script. But this is still have risk if users specify more than one container in the worker spec, in which we don't which is the main container, which is the sidecar...

[k8s-ci-robot]

New changes are detected. LGTM label has been removed.

[ywskycn]

Add a new PR which exposes the mainContainer as a field in job level, instead of the previous operator-level.

If users don't specify mainContainer, the exec command would be kubectl exec {POD_NAME} -- /bin/sh -c "$*", and it will run against the default container. If specified, the command would be kubectl exec ${POD_NAME} --container ${mainContainer} -- /bin/sh -c "$*".

[terrytangyuan]

Looks good to me. Thanks!