[java-jaxrs] Fix paths when useTags=true is used

[jmini]

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, 3.1.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

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

Description of the PR

Detected during test of #215.
When you use the useTags boolean option with the jaxrs-jersey generator, there is a problem with the paths.

Example spec: tags.yaml (added to this PR as test case)

Then the endpoints are:

http://localhost:8080/v2/Tag1/op1
http://localhost:8080/v2/Tag2/op2
http://localhost:8080/v2/Tag2/op3
http://localhost:8080/v2/Default/op4
http://localhost:8080/v2/Group4/op5
http://localhost:8080/v2/Group4/op6

They are driven by the tags (and tags name).

They should be as specified in the paths section:

http://localhost:8080/v2/group1/op1
http://localhost:8080/v2/group1/op2
http://localhost:8080/v2/group2/op3
http://localhost:8080/v2/group3/op4
http://localhost:8080/v2/group4/op5
http://localhost:8080/v2/group4/op6

This PR introduces a new variable accessible in the JaxRS templates: {{commonPath}}.
This is because the {{baseName}} when tags are used can not be used in the top level @Path annotation value.
{{commonPath}} contains the path common to all operations of the generated groups.

A unit test to ensure that nothing was changed cases where useTags is set to false (default behavior) is also added to the PR.

[wing328]

The change and the updated samples look good to me (but I didn't test it locally due to time constraint)