Preset DW strings for copy_in and copy_out

[jameshcorbett]

Preset DW strings can now be set in a config file, but the original use case was for #DW jobdw strings. I was thinking that it might be nice to do preset DW strings that do copy in and out.

I'm imagining flux alloc ... -S dw=xfs_large_copy where xfs_large_copy expands to

#DW jobdw type=xfs capacity=256GiB name=project1
#DW copy_in source=/p/lustre1/$USER/dir_in destination=$DW_JOB_project1/
#DW copy_out source=$DW_JOB_project1/ destination=/p/lustre1/$USER/dir_out/

But either the flux-coral2 plugins would have to expand that $USER field, or DWS would.