ENH: Lightweight node cache checking

[oesteban]

Generating the hashvalue when outputs are not ready at cache check stage
when the node's directory does not exist (or no results file is in
there) leads to #3014.

This PR preempts those problems by delaying the hashval calculation.

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]

Codecov Report

Merging #3026 into master will decrease coverage by 3.33%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3026      +/-   ##
==========================================
- Coverage   67.51%   64.18%   -3.34%     
==========================================
  Files         344      342       -2     
  Lines       44045    43991      -54     
  Branches     5550     5547       -3     
==========================================
- Hits        29739    28237    -1502     
- Misses      13567    14628    +1061     
- Partials      739     1126     +387

Flag	Coverage Δ
#smoketests	?
#unittests	64.18% <100%> (-0.78%)	⬇️

Impacted Files	Coverage Δ
nipype/pipeline/engine/nodes.py	77.35% <100%> (-6.76%)	⬇️
nipype/interfaces/nilearn.py	40% <0%> (-56.67%)	⬇️
nipype/utils/spm_docs.py	25.92% <0%> (-44.45%)	⬇️
nipype/interfaces/freesurfer/base.py	50% <0%> (-30.51%)	⬇️
nipype/utils/logger.py	59.7% <0%> (-29.86%)	⬇️
nipype/algorithms/rapidart.py	35% <0%> (-29.42%)	⬇️
nipype/interfaces/spm/base.py	57.94% <0%> (-29.14%)	⬇️
nipype/utils/provenance.py	55.73% <0%> (-28.35%)	⬇️
nipype/interfaces/fsl/model.py	55.26% <0%> (-25.35%)	⬇️
nipype/testing/fixtures.py	77.33% <0%> (-21.34%)	⬇️
... and 43 more

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[oesteban]

Travis seems a bit under the weather today. I'll go ahead and merge, making sure master passes all tests.