Fixed yaml parsing error for !GetAtt

[dtimberlake]

When parsing !GetAtt, attributes should not be split on periods.

Example:

Original:

!GetAtt MyStack.Outputs.Arn

Parses to:

{ "Fn::GetAtt": [ "MyStack", "Outputs", "Arn" ] }

Should be:

{ "Fn::GetAtt": [ "MyStack", "Outputs.Arn" ] }

[jamesls]

Good catch, thanks for the pull request.