No longer generating ssl cert when one is already specified

[mrsharpoblunto]

What kind of change does this PR introduce?
Small startup time optimization

Did you add or update the examples/?

Summary
Skip the generation of a self signed SSL cert if an existing cert is already specified in the https options. The self signed SSL cert never gets used if you specify a cert in the options anyway, so theres not much point generating it in this case

Does this PR introduce a breaking change?
No

Other information

[jsf-clabot]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #1036 into master will decrease coverage by 0.3%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1036      +/-   ##
==========================================
- Coverage   72.25%   71.94%   -0.31%     
==========================================
  Files           4        4              
  Lines         465      467       +2     
  Branches      139      140       +1     
==========================================
  Hits          336      336              
- Misses        129      131       +2

Impacted Files	Coverage Δ
lib/Server.js	79.33% <0%> (-0.49%)	⬇️

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[shellscape]

@mrsharpoblunto thanks for the PR! please sign the CLA and we can move forward

[mrsharpoblunto]

@shellscape Signed the CLA