Revert "Run all dsvm jobs from juno onwards on trusty"

This reverts commit c76f930c09b57ac134d8610ed5fde6d3b802806b.

This is necessary as there is a Jenkings gearman-plugin bug that causes
a node to register all available labels for a job even if it is capable
of serving only one label. This bug prevents zuul from properly
selectiong nodes as desired by this change.

Change-Id: I035d31a91c4ca1a8366316b7ff422f4a2b03622c