NO-JIRA: Set customizable TNA NUM_WORKERS in common.sh

[fracappa]

Changing NUM_WORKERS env var in TNA to be customizable.
In this way TNA clusters with day-0 workers could be installed using dev-scripts

[openshift-ci]

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[openshift-ci]

Hi @fracappa. Thanks for your PR.

I'm waiting for a openshift-metal3 member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

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