[BEAM-6418] Execute Flink tests serially on Jenkins to avoid memory issues
[BEAM-6418] Execute Flink tests serially on Jenkins to avoid memory issues

We had previously disabled Flink tests for 1.6 and 1.7 due to memory issues when
they executed in parallel. This lets them execute one after another on Jenkins.

RyanSkraba/beam,lukecwik/incubator-beam,RyanSkraba/beam,robertwb/incubator-beam,robertwb/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,RyanSkraba/beam,robertwb/incubator-beam,apache/beam,apache/beam,chamikaramj/beam,markflyhigh/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,chamikaramj/beam,RyanSkraba/beam,iemejia/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,lukecwik/incubator-beam,iemejia/incubator-beam,markflyhigh/incubator-beam,lukecwik/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,robertwb/incubator-beam,markflyhigh/incubator-beam,robertwb/incubator-beam,lukecwik/incubator-beam,markflyhigh/incubator-beam,RyanSkraba/beam,lukecwik/incubator-beam,robertwb/incubator-beam,RyanSkraba/beam,lukecwik/incubator-beam,apache/beam,markflyhigh/incubator-beam,lukecwik/incubator-beam,chamikaramj/beam,chamikaramj/beam,lukecwik/incubator-beam,robertwb/incubator-beam,lukecwik/incubator-beam,apache/beam,apache/beam,markflyhigh/incubator-beam,apache/beam,RyanSkraba/beam,markflyhigh/incubator-beam,chamikaramj/beam,apache/beam,chamikaramj/beam,apache/beam,apache/beam,apache/beam,apache/beam