Issue 83: Support terraform fmt

[kmanning]

• Issue Support terraform fmt #83 - add terraform fmt as part of the TerraformValidateStage.
• Support optional recursive directory check
• Support displaying diffs
• Ensure support for both terraform 0.11.x and 0.12.x

[codecov-commenter]

Codecov Report

Merging #298 into master will increase coverage by 1.89%.
The diff coverage is 86.00%.

@@            Coverage Diff             @@
##           master     #298      +/-   ##
==========================================
+ Coverage   76.54%   78.43%   +1.89%     
==========================================
  Files          39       41       +2     
  Lines        1006     1048      +42     
  Branches      235      244       +9     
==========================================
+ Hits          770      822      +52     
+ Misses        122      105      -17     
- Partials      114      121       +7     

Impacted Files	Coverage Δ
src/TerraformValidateStage.groovy	81.81% <ø> (+54.54%)	⬆️
src/ValidateFormatPlugin.groovy	71.42% <71.42%> (ø)
src/TerraformFormatCommand.groovy	86.11% <86.11%> (ø)
src/TerraformPlugin.groovy	95.00% <100.00%> (+4.09%)	⬆️
src/TerraformPluginVersion12.groovy	86.66% <100.00%> (+28.33%)	⬆️

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