Adding coverage command back

[vmx]

I think it would make sense to add the coverage command back. This way we can swap out the converage tool we are using without the need to update all the CI files.

If there's an agreement on that we want to have the command back, I'm happy to provide a PR.

[mikeal]

I’d really like a aegir coverage —100 mode where it configured the coverage checker to 100 in all categories. Would save a lot of time and annoyance :)