[BEAM-3089] Fix job parallelism resolution

[grzegorz8]

DefaultParallelismFactory has been removed since it ignores parallelism value
which is specified at job submission. Instead, FlinkPipelineOptions#parallelism
defaults to -1, which means the execution environment's parallelism should
be used.

Now the precedence is as follows:

1. the value specified directly in the source code
2. the value passed as a Flink Runner parameter at job submission
3. the value passed as a Flink CLI / Flink UI parameter at job submission
4. the default value for Flink cluster

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[grzegorz8]

Some tests implicitly assume that parallelism=1, so I had to explicitly specify it. I can try to refactor them to work correctly regardless of the parallelism, if needed.

[aljoscha]

retest this please

[aljoscha]

Run Flink ValidatesRunner

[grzegorz8]

retest this please

[grzegorz8]

retest this please

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