[FIX] MapNodes fail when MultiProcPlugin passed by instance

[oesteban]

When calling workflow.run(plugin=MultiProcPlugin()), MapNodes are set the use_plugin member that is not pickleable if plugin is a MultiProcPlugin object.

Summary

Fixes # .

List of changes proposed in this PR (pull-request)

Acknowledgment

• (Mandatory) I acknowledge that this contribution will be available under the Apache 2 license.

[codecov-io]

Codecov Report

Merging #2786 into master will decrease coverage by 3.38%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2786      +/-   ##
==========================================
- Coverage   67.49%   64.11%   -3.39%     
==========================================
  Files         341      339       -2     
  Lines       43285    43236      -49     
  Branches     5364     5361       -3     
==========================================
- Hits        29214    27719    -1495     
- Misses      13369    14447    +1078     
- Partials      702     1070     +368

Flag	Coverage Δ
#smoketests	?
#unittests	64.11% <100%> (-0.78%)	⬇️

Impacted Files	Coverage Δ
nipype/pipeline/engine/workflows.py	72.37% <100%> (-6.71%)	⬇️
nipype/interfaces/nilearn.py	40% <0%> (-56.67%)	⬇️
nipype/utils/spm_docs.py	25.92% <0%> (-44.45%)	⬇️
nipype/interfaces/freesurfer/base.py	49.59% <0%> (-30.9%)	⬇️
nipype/utils/logger.py	59.7% <0%> (-29.86%)	⬇️
nipype/algorithms/rapidart.py	35.39% <0%> (-29.21%)	⬇️
nipype/interfaces/spm/base.py	58.41% <0%> (-29.05%)	⬇️
nipype/utils/provenance.py	55.73% <0%> (-28.99%)	⬇️
nipype/interfaces/fsl/model.py	55.26% <0%> (-25.35%)	⬇️
nipype/testing/fixtures.py	77.33% <0%> (-21.34%)	⬇️
... and 42 more

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[oesteban]

Given the very low risk of this patch, I'll go ahead and merge it.