FIX: Avoid excessive etelemetry pings

[mgxd]

In cases where check_latest_version returned None, each time
BaseInterface.__init__ was called would lead to another server ping.
This avoids this loop by falling to a string, n/a, if the server fails
to produce a version.

Summary

Fixes #3483 .

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

[codecov]

Codecov Report

Merging #3484 (5b9f3a7) into master (71c7a7d) will decrease coverage by 1.62%.
The diff coverage is 50.00%.

@@ Coverage Diff @@ ## master #3484 +/- ## ========================================== - Coverage 65.25% 63.63% -1.63%  ========================================== Files 309 309 Lines 40848 40848 Branches 5378 5378 ========================================== - Hits 26657 25993 -664  - Misses 13117 13830 +713  + Partials 1074 1025 -49 

Flag	Coverage Δ
unittests	?

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Impacted Files	Coverage Δ
nipype/__init__.py	53.19% <0.00%> (ø)
nipype/interfaces/base/core.py	85.63% <100.00%> (-2.34%)	⬇️
nipype/workflows/__init__.py	47.05% <0.00%> (-52.95%)	⬇️
nipype/utils/spm_docs.py	20.00% <0.00%> (-48.00%)	⬇️
nipype/testing/fixtures.py	57.14% <0.00%> (-41.56%)	⬇️
nipype/interfaces/freesurfer/base.py	48.38% <0.00%> (-28.23%)	⬇️
nipype/utils/logger.py	56.92% <0.00%> (-27.70%)	⬇️
nipype/interfaces/fsl/maths.py	79.06% <0.00%> (-19.19%)	⬇️
nipype/interfaces/dcm2nii.py	50.00% <0.00%> (-15.28%)	⬇️
nipype/interfaces/fsl/base.py	67.02% <0.00%> (-13.83%)	⬇️
... and 22 more

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