upgrade script to use pyton3

[mbiarnes]

in case the art-dashboard-server container runs in bash mode (needed i.e. if it is required to do a kinit inside the container) the script ./start-cmd.sh has to be started. Since all Dockerfiles install or update python3 the script has to be updated to use the same version. If not the python is not recognized.

[openshift-ci]

@mbiarnes: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/security	e6c35c9	link	false	/test security

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[ashwindasr]

This change will be for development environment only

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ashwindasr

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