vagrant: Fix CentOS6 mirrorlink issue

[Willsparker]

ref: #1745 , #1756

Fixes the issue that the repos for CentOS6 are outdated due to C6 coming to the end of it's lifespan November 2020. This PR just enables the baseURL, and changes it to CentOS's vault URL.

As mentioned in #1756 , this may not be the best place for this fix. It is affecting the CentOS6 Docker containers too (#1745)- however the Vagrantfiles certainly do need it, as it can't install libselinux-python without it.

Checklist

• commit message has one of the standard prefixes
• playbook changes run through VPC or QPC .
  ^ I'd love to, but unfortunately, there's an issue with the machine that runs it, currently. I've tested this locally though and it does work.

[gdams]

@Willsparker I've updated your PR to make these changes to the docker image too

[Willsparker]

Oh awesome, thanks :-)

Fixes: #1745

[karianna]

Can't be any worse - so here goes.