Improve network usage by installing a proxy on every hosts

By using transparent proxy on every host we could:

• Save a lot of network bandwidth, because traffic would go through KVM directly. It would avoid downloading not only provisioning scripts, dependencies of pyside, Coin and QtWebEngine (which download data from Internet during _build_), but also with https://codereview.qt-project.org/#/c/205560/6 every job in the CI.
• Improve speed, data transfer inside KVM is above 30Gb/s of _real_ transfer while we are sitting 2x1Gb/s link, increasing it even to 10Gb/s would not give us close performance
• Improve reliability, we know that our network is not stable enough, reducing it usage means reducing chances of hitting problems.

In extreme world we would install proxy certificates and allow it to de-crypt and cache https transfers.

Alternatively we can set non transparent proxy and enforce it usage.