Rename main step name to "main"

[Tomcli]

Which issue is resolved by this Pull Request:
Resolves #154

Description of your changes:
Since Tekton allows multiple step spec in a task whereas Argo only allows one container spec per task, it's not obvious for Tekton to tell which step is from the main ContainerOp. Therefore, we want to rename all the containerOp step name to "main" to distinguish between the initContainers and main step.

This has no impact to our existing examples because we just change the container name for logging with the API.

Environment tested:

• Python Version (use python --version): 3.6.4
• Tekton Version (use tkn version): 0.11.3
• Kubernetes Version (use kubectl version): 1.16
• OS (e.g. from /etc/os-release):

[kubeflow-bot]

This change is 

[drewbutlerbb4]

Can we do the same for Task templates created for ResourceOps

In the compiler: _op_to_template.py

In the testdata: resourceop_basic.yaml

[Tomcli]

Thanks @drewbutlerbb4, I addressed the comment.

[drewbutlerbb4]

/lgtm

[animeshsingh]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: animeshsingh

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 [animeshsingh]

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