[xenial] Support running CI staging tests against both 14.04 and 16.04

[eloquence]

To better detect bugs during the transition period from Ubuntu 14.04 to 16.04, we should run two staging workflows in CI until the transition is complete: a Trusty workflow and a Xenial workflow.

Investigate whether per-PR CI performance hit is acceptable, otherwise configure to run one of the two (initially Xenial) nightly.

Part of #3204.