Added server variable support

Created by: TiFu

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• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

See #793 (closed)

I implemented a solution similar to authMethods in supportingFiles JSON (see generated json below).

Spec: https://gist.github.com/TiFu/0934b362f25c66bc2b758f33e15520fc Generated Supporting Files JSON: https://gist.github.com/TiFu/919a8888837e4e24fd32a05dc8d289f9

Generated under the root object (use -DdebugSupportingFiles to print the full mustache template used):

 "servers" : [ {
    "url" : "http://petstore.swagger.io:{port}/{basePath}",
    "description" : "The production API server",
    "variables" : [ {
      "name" : "port",
      "defaultValue" : "8443",
      "enumValues" : [ "8443", "443" ]
    }, {
      "name" : "basePath",
      "defaultValue" : "v2"
    } ]
  }, {
    "url" : "http://petstore.swagger.io",
    "description" : "The production API server",
    "variables" : [ ]
  } ],

Question

Do I have to regenerate any samples? This is a non-breaking change because I only added a new property to the JSON (which is passed to mustache) for supportingFiles.

CC @wing328 @jmini