Task option concurrent=False

[BoPeng]

With #1125 we changed the default concurrent input option to True. This however broke a test

input: for_each=dict(i=range(5))
task: concurrent=False

because when the substeps are executed in parallel, the tasks are started from individual workers so concurrent=False is ignored.

Actually, even

input: for_each=dict(i=range(5)), concurrent=False
task: 

does not fix the problem because even when the substeps are executed sequentially, the tasks are executed in parallel after they are submitted.

So we should add concurrent=False to input when this option is specified for task.

input: for_each=dict(i=range(5))
task: concurrent=False