Allow to source --multi output

[Tof1973]

What type of PR is this?

feature

What this PR does / Why we need it?

When using login --multi, the output cannot be sourced directly.
With this change we will be able to run:

source <(ocm-backplane login multi xxx)

Which Jira/Github issue(s) does this PR fix?

Resolves #

Special notes for your reviewer

Pre-checks (if applicable)

• [ x] Ran unit tests locally
• [ x] Validated the changes in a cluster
• [ x] Included documentation changes with PR

[openshift-ci]

@Tof1973: all tests passed!

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[codecov-commenter]

Codecov Report

Merging #289 (25b2d1d) into main (da7e96c) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #289   +/-   ##
=======================================
  Coverage   46.91%   46.91%           
=======================================
  Files          53       53           
  Lines        3568     3568           
=======================================
  Hits         1674     1674           
  Misses       1633     1633           
  Partials      261      261           

Files	Coverage Δ
pkg/login/kubeConfig.go	59.37% <100.00%> (ø)

[samanthajayasinghe]

Thanks for improving the usage of multi @Tof1973 ,
FYI , You can use backplane session for multi-cluster login where you don't need to set kubecong manually : ref

[samanthajayasinghe]

/lgtm

[feichashao]

/approve

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: feichashao, samanthajayasinghe, Tof1973

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