fix: openapi postprocess bug in request models with security definitions

[praneetap]

Issue #, if available:
securityDefinitions don't get changed to components/securitySchemes if request models are defined, and this breaks the cloudformation deployment.
Description of changes:
Moved openapi postprocess to a single function.
Description of how you validated changes:
Wrote unit tests
Deployed the following template successfully on cfn

Resources:
  MyApi:
    Type: AWS::Serverless::Api
    Properties:
      OpenApiVersion: 3.0.1
      StageName: Prod
      Models:
        User:
          type: object
          properties:
            username:
              type: string

  MyFunction:
    Type: AWS::Serverless::Function
    Properties:
      Handler: index.handler
      Runtime: nodejs8.10
      InlineCode: |
        exports.handler = async (event, context, callback) => {
          return {
            statusCode: 200,
            body: 'Success'
          }
        }
      Events:
        Iam:
          Type: Api
          Properties:
            RequestModel:
              Model: User
              Required: true
            RestApiId:
              Ref: MyApi
            Method: get
            Path: /iam
            Auth:
              Authorizer: AWS_IAM
Outputs:
  ApiUrl:
    Description: "API endpoint URL for Prod environment"
    Value:
      Fn::Sub: 'https://${MyApi}.execute-api.${AWS::Region}.amazonaws.com/Prod/'

Checklist:

• Write/update tests
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected
• Add/update example to examples/2016-10-31

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

Merging #990 into release/v1.13.0 will increase coverage by 0.14%.
The diff coverage is 87.5%.

@@                 Coverage Diff                 @@
##           release/v1.13.0     #990      +/-   ##
===================================================
+ Coverage            94.71%   94.86%   +0.14%     
===================================================
  Files                   69       69              
  Lines                 3160     3152       -8     
  Branches               606      602       -4     
===================================================
- Hits                  2993     2990       -3     
+ Misses                  85       84       -1     
+ Partials                82       78       -4

Impacted Files	Coverage Δ
samtranslator/model/api/api_generator.py	95.33% <87.5%> (+1.74%)	⬆️

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