Island: Set terminate signal for duplicate agents

[cakekoa]

What does this PR do?

Fixes part of #2817.

If more than one agent is running on a given machine, the first agent shall be allowed to run, and the rest shall be given a terminate signal.

PR Checklist

• Have you added an explanation of what your changes do and why you'd like to include them?
• Is the TravisCI build passing?
• Was the CHANGELOG.md updated to reflect the changes?
• Was the documentation framework updated to reflect the changes?
• Have you checked that you haven't introduced any duplicate code?

Testing Checklist

• Added relevant unit tests?
• Have you successfully tested your changes locally? Elaborate:

  Tested by running unit tests

• If applicable, add screenshots or log transcripts of the feature working

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.02 🎉

Comparison is base (b454c88) 67.26% compared to head (3c66438) 67.28%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3058      +/-   ##
===========================================
+ Coverage    67.26%   67.28%   +0.02%     
===========================================
  Files          439      439              
  Lines        12636    12644       +8     
===========================================
+ Hits          8499     8507       +8     
  Misses        4137     4137              

Impacted Files	Coverage Δ
...monkey_island/cc/services/agent_signals_service.py	100.00% <0.00%> (ø)

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