Bluetooth: Mesh: Add build of provisioner sample to CI

Adds a test case listing in sample.yaml of the mesh_provisioner sample,
building on qemu and the Nordic nRF51 DK to ensure that the provisioner
role is built in PRs.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>