Move VPC Tenancy check to AllowedValues

[fatbasstard]

Issue #50, if available:

Moved over the VPC Tenancy value check to the generic Allowed Values setup. Along the way:

• Corrected/improved Custom Resource check
• Moved the Ref check into the ValueRefGetAtt (E3008) rule
• Added move test values in the "bad" template

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[codecov-io]

Codecov Report

Merging #540 into master will increase coverage by 0.09%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #540      +/-   ##
==========================================
+ Coverage   84.44%   84.54%   +0.09%     
==========================================
  Files         110      110              
  Lines        5946     5925      -21     
  Branches     1346     1342       -4     
==========================================
- Hits         5021     5009      -12     
+ Misses        609      604       -5     
+ Partials      316      312       -4

Impacted Files	Coverage Δ
src/cfnlint/rules/resources/ectwo/Vpc.py	76.31% <33.33%> (+5.12%)	⬆️
...cfnlint/rules/resources/properties/AllowedValue.py	97.01% <0%> (+1.49%)	⬆️

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