Allow specifying/overriding the parent in the pom file for Java and Spring

Created by: mwoodland

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\.
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Description of the PR

Add support for overriding/specifying the parent in the generated pom file. issue #1131

The run-all-petstore.cmd command did't work as some of the scripts it calls are looking for jars in swagger-codegen-cli directory - hence why I haven't checked "ran the schell script" above.

I have checked using the Spring generator while specifying none of the parent properties, all of them and one of them.

@bbdouglas (2017/07) @JFCote (2017/08) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01)