OpenstackLatentWorker: Try to cleanup existing worker(s) before starting new

[cmouse]

Contributor Checklist:

• I have updated the unit tests
• I have created a file in the master/buildbot/newsfragments directory (and read the README.txt in that directory)
• I have updated the appropriate documentation
• I have tested my change

[codecov]

Codecov Report

Merging #5947 (7b9f13a) into master (2742d5c) will decrease coverage by 0.15%.
The diff coverage is 100.00%.

❗ Current head 7b9f13a differs from pull request most recent head 64b8759. Consider uploading reports for the commit 64b8759 to get more accurate results

@@            Coverage Diff             @@
##           master    #5947      +/-   ##
==========================================
- Coverage   91.81%   91.65%   -0.16%     
==========================================
  Files         340      340              
  Lines       36347    36357      +10     
==========================================
- Hits        33371    33323      -48     
- Misses       2976     3034      +58     

Impacted Files	Coverage Δ
master/buildbot/worker/openstack.py	94.94% <100.00%> (+0.26%)	⬆️
master/buildbot/__init__.py	76.92% <0.00%> (-15.39%)	⬇️
worker/buildbot_worker/__init__.py	77.61% <0.00%> (-14.93%)	⬇️
.../migrate/versions/048_change_properties_to_text.py	87.50% <0.00%> (-12.50%)	⬇️
...migrate/versions/046_mysql_innodb_compatibility.py	89.47% <0.00%> (-10.53%)	⬇️
master/buildbot/db/test_results.py	89.54% <0.00%> (-6.54%)	⬇️
master/buildbot/db/enginestrategy.py	69.17% <0.00%> (-5.48%)	⬇️
...dbot/db/migrate/versions/052_cascading_set_null.py	94.00% <0.00%> (-4.00%)	⬇️
master/buildbot/process/botmaster.py	94.64% <0.00%> (-3.58%)	⬇️
master/buildbot/db/connector.py	96.51% <0.00%> (-3.49%)	⬇️
... and 1 more

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[p12tic]

This PR looks good in principle, just a couple of nitpicks are left.

[p12tic]

@cmouse Looks good. Could you check out the formatting? pylint and isort tests are failing.

[cmouse]

Didn't I fix those already ...