[main] add support to configure dashboard air gap image

[jkandasa]

Changes

• adding offline image support for dashboard
• Partially implements, Support offline installation #1894
• fixes: Install tekton operator in air-gapped network #1910

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Run make test lint before submitting a PR
• Includes tests (if functionality changed/added)
• Includes docs (if user facing)
• Commit messages follow commit message best practices

See the contribution guide for more details.

Release Notes

Support added to dashboard air gap image, the image can be replaced on the operator deployment environment variable `IMAGE_DASHBOARD_TEKTON_DASHBOARD`

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-operator-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/kubernetes/tektondashboard/transform.go	0.0%	85.7%	85.7

[piyush-garg]

/approve

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: piyush-garg

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 [piyush-garg]

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

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-operator-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/kubernetes/tektondashboard/transform.go	0.0%	85.7%	85.7

[jkhelil]

/lgtm

[tekton-robot]

@jkhelil: changing LGTM is restricted to collaborators

In response to this:

/lgtm

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[PuneetPunamiya]

/lgtm