run gather in background

[SeanMooney]

run_bg can only be used to run executables in the backgorund
not bash functions. This change adds a wait_for_bg_slot funciton
and refactors the sos report gathering to run in parallel up to
the concurrancy limit.

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[bogdando]

/lgtm

[Akrog]

I think it would be good to have "why" the run_bg function is not appropriate in the commit message.

[gibizer]

I've opened a tracker Jira for this https://issues.redhat.com/browse/OSPRH-7830
The bug only appears if the strict error checking is enabled in bash, but we cannot enable that globally in the scripts due to other error handling shortcomings.

[Akrog]

Thanks @gibizer I think that context is needed in the commit message and the PR description.
From the existing content I thought there was a bad behavior of the script, which doesn't seem to be the case.
Though I agree with the intent of the PR to make changes to enable strict error checking.