cmd/createdkg: add existing cluster definition check

[dB2510]

Add a check for existing cluster-definition.json file. This will error out and leave the current file intact.

category: bug
ticket: #1062

[codecov]

Codecov Report

Merging #1111 (1e95104) into main (50e6b5c) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1111      +/-   ##
==========================================
+ Coverage   52.98%   53.01%   +0.03%     
==========================================
  Files         128      128              
  Lines       14482    14485       +3     
==========================================
+ Hits         7673     7679       +6     
+ Misses       5707     5704       -3     
  Partials     1102     1102              

Impacted Files	Coverage Δ
cmd/createdkg.go	70.21% <100.00%> (+0.98%)	⬆️
app/app.go	59.48% <0.00%> (-1.78%)	⬇️
core/scheduler/scheduler.go	74.04% <0.00%> (+0.54%)	⬆️
core/dutydb/memory.go	70.89% <0.00%> (+1.11%)	⬆️
dkg/transport.go	66.66% <0.00%> (+12.96%)	⬆️

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