Allow no Q2T servers to be specified on a bootstrap node

[SatpalSandhu61]

Fixes #790
Add new config item "bootstrapNode" which facilitates setup of a bootstrap node with no Q2T serverConfig entries.

[codecov-commenter]

Codecov Report

Merging #1084 into master will decrease coverage by 0.01%.
The diff coverage is 96.42%.

@@             Coverage Diff              @@
##             master    #1084      +/-   ##
============================================
- Coverage     99.41%   99.39%   -0.02%     
- Complexity     2258     2261       +3     
============================================
  Files           349      349              
  Lines          6278     6289      +11     
  Branches        358      360       +2     
============================================
+ Hits           6241     6251      +10     
  Misses            5        5              
- Partials         32       33       +1     

Flag	Coverage Δ	Complexity Δ
#unittests	99.39% <96.42%> (-0.02%)	2261.00 <12.00> (+3.00)	⬇️

Impacted Files	Coverage Δ	Complexity Δ
...era/config/constraints/ServerConfigsValidator.java	96.87% <95.83%> (-3.13%)	9.00 <8.00> (+2.00)	⬇️
...rc/main/java/com/quorum/tessera/config/Config.java	100.00% <100.00%> (ø)	34.00 <4.00> (+1.00)

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[Krish1979]

LFTM