Apply alive test preferences if one method was selected.

[jjnicola]

What:
The selected alive test option overwrite the default settings.

Why:
The issue was that the preferences where added twice, but always
the default ones were used because were found first.

How:
Run a scan with some alive test method different to the default one. An easy tests is to select "consider alive" against a dead hosts.

Checklist:

• Tests
• CHANGELOG Entry

[codecov]

Codecov Report

Merging #334 into ospd-openvas-20.08 will decrease coverage by 0.72%.
The diff coverage is 62.50%.

@@                  Coverage Diff                   @@
##           ospd-openvas-20.08     #334      +/-   ##
======================================================
- Coverage               82.71%   81.99%   -0.73%     
======================================================
  Files                       9        9              
  Lines                    1481     1516      +35     
======================================================
+ Hits                     1225     1243      +18     
- Misses                    256      273      +17     

Impacted Files	Coverage Δ
ospd_openvas/daemon.py	62.01% <31.81%> (-0.90%)	⬇️
ospd_openvas/db.py	96.24% <57.14%> (-3.36%)	⬇️
ospd_openvas/preferencehandler.py	87.88% <86.36%> (+0.78%)	⬆️
ospd_openvas/__version__.py	100.00% <100.00%> (ø)
ospd_openvas/lock.py	85.96% <100.00%> (+0.25%)	⬆️
ospd_openvas/nvticache.py	98.49% <100.00%> (+0.01%)	⬆️
ospd_openvas/openvas.py	100.00% <100.00%> (ø)

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