Update SharedCodegen to fallback to title usage if x-model is not present

[macisamuele]

The current plugin does use x-model field for model name determination.

According to the swagger specification title should be used to define the model name.
Internally, in Yelp, we heavily use x-model custom vendor extension to be able to define names also for objects that do not allow the presence of title.

The goal of this PR is to ensure that title is honoured if present and used as fallback in case x-model is not present.

Let's get the following specs

swagger: "2.0",
info: {
  title: Test
  version: 1.0.0
paths: {},
definitions:
  def1:
    type: object
  def2:
    type: object
    x-model: model2
  def3:
    title: title3
    type: object
  def4:
    title: title4
    type: object
    x-model: model4

Before this PR the generated models would be:

• def1 for #/definitions/def1
• model2 for #/definitions/def2
• def3 for #/definitions/def3
• model4 for #/definitions/def4
  After this PR the generated models would be:
• def1 for #/definitions/def1
• model2 for #/definitions/def2
• title3 for #/definitions/def3 DIFFERENT
• model4 for #/definitions/def4