topotests: backward compatibility fix

[rzalamena]

Summary

Allow old topologies to use default parameters from pytest.ini.

Related issue

Fixes #3556

[LabN-CI]

💚 Basic BGPD CI results: SUCCESS, 0 tests failed

Results table

_	_
Result	SUCCESS git merge/4502 04ce2b9
Date	06/11/2019
Start	15:06:56
Finish	15:29:46
Run-Time	22:50
Total	1813
Pass	1813
Fail	0
Valgrind-Errors	0
Valgrind-Loss	0
Details	vncregress-2019-06-11-15:06:56.txt
Log	autoscript-2019-06-11-15:07:59.log.bz2
Memory	436 426 360

For details, please contact louberger

[NetDEF-CI]

Continuous Integration Result: SUCCESSFUL

Congratulations, this patch passed basic tests

Tested-by: NetDEF / OpenSourceRouting.org CI System

CI System Testrun URL: https://ci1.netdef.org/browse/FRR-FRRPULLREQ-8003/

This is a comment from an automated CI system.
For questions and feedback in regards to this CI system, please feel free to email
Martin Winter - mwinter (at) opensourcerouting.org.

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CLANG Static Analyzer Summary

• Github Pull Request 4502, comparing to Git base SHA afbdfbb

No Changes in Static Analysis warnings compared to base

4 Static Analyzer issues remaining.

See details at
https://ci1.netdef.org/browse/FRR-FRRPULLREQ-8003/artifact/shared/static_analysis/index.html

[rwestphal]

Thanks for working on this. I confirm this fixes #3556.