edited CentralizedArchitecture template to use Fn::Sub instead of Fn:…

[pmankad96]

Submitted following changes to CentralizedArchitecture CF templates:

• Replaced Fn::Join with Fn::Sub for Tag (Values)
• Fixed wrong route entries in NAT GW route tables
• Added custom resource to enable Transit Gateway appliance mode for Appliance VPC.

Issue #, if available:

Description of changes:

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