refactor: only define usr-host volume if to be mounted by a container

[consideRatio]

This PR makes a usr-host Volume be conditionally added, side by side with the conditionally added VolumeMount entry referencing it.

I think the current behavior of declaring a volume that isn't mounted will be fully functional, and this fix just makes it easier to understand the workload by not including an unused volume.

[fntlnz]

It makes sense @consideRatio this was a mistake in the first instance, thanks for fixing it!

[consideRatio]

I rebased this PR on the current master branch. I think that should resolve the test failures previously observed where ubuntu-16.04 still was used.

[fntlnz]

Thank you @consideRatio !