[Spec] adds XmlItem model and route for xml testing

Created by: spacether

PR checklist

• Read the contribution guidelines.
• 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\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language. @wing328 @jimschubert @cbornet @jaz-ah @ackintosh @JFCote @jmini

Description of the PR

1. This PR adds the model XmlItem and a route that uses it to the swagger 2.0 sample spec. It also includes 6 xml mime types that are produced + consumed in that route. Downstream clients can use this model and that route for their xml tests.
2. It adds an Api suffix on the java rest-assured client to pass CircleCi tests
3. It pegs the versions of pytest and pytest-cov to fix and pass TravisCI tests

The examples currently in the spec do not include all xml attributes. The XmlItem model includes all xml attributes:

• name
• attribute
• wrapped
• prefix

Issue Closure

When merged this PR will close these issues:

• [BUG] swagger 2.0 spec is missing an xml example with all xml fields https://github.com/OpenAPITools/openapi-generator/issues/1807
• [CI] Python Tests Failing in TravisCI https://github.com/OpenAPITools/openapi-generator/issues/1832

Note: this is the cleaned up version of this PR: https://github.com/OpenAPITools/openapi-generator/pull/1808