update trigger to match new workflow label

after upgrading gerrit to ver 2.8, review.o.o will have a new label called 'Workflow'
that combines approved and wip labels. Updating the trigger to match the label.

We need to keep the 'approved' label so it will continue to work with gerrit ver 2.4.4

The gerrit workflow label is defined as:

[label "Workflow"]
        function = MaxWithBlock
        value = -1 Work in progress
        value =  0 Ready for reviews
        value = +1 Approved

Note: This same change has already been applied and tested on review-dev.o.o
Ieb7e2dde49cd64f2aa3dbf939c391492d73ed886

Partial-Bug: #1082781
Change-Id: I849eca5f5c20a21ad0007541ea60c82e2da343e0
Merge "update trigger to match new workflow label"