[BEAM-2956] Make the DataflowRunner correctly report that it is using the Dataflow distribution.

[lukecwik]

Note that this test will not pass until https://github.com/apache/beam/pull/3849 is merged and the Apache Beam nightly 2.2.0-SNAPSHOT is updated.

[lukecwik]

R: @kennknowles

[lukecwik]

retest this please

[lukecwik]

R: @tgroh

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