Prepare release 3.0.2 (OpenAPITools#337)

Prepare 3.0.1 release (OpenAPITools#280)

* add new bash script for release version update

* update version using the bash script

* Fix shippable build

Modify `artifactId` of `CI/pom.xml.shippable` to be `openapi-generator-shippable-pom`

* Comment ensure-up-to-date

* add shippable to the release script

* Updates README based on new release changes (OpenAPITools#271)

The release management changes moved from maven-publish (newer plugin)
to maven (older plugin, only one that works currently with signing).
This updates docs in the samples/local-spec project with current
directions.

Also:

* Includes sonatype releases/snapshots on repo lookup
* Adds openApiGeneratorVersion property

update docker file to use stable JAR