Verifier docker image support for custom stubs classifier

Adds the possibility to provide custom stubs classifier. Might be useful to serve as some kind of tag for the stubs published.

docker/spring-cloud-contract-docker/project/build.gradle

@@ -60,6 +60,7 @@ test {
 contracts {
 	baseClassForTests = "contracts.RestBase"
 	testMode = "EXPLICIT"
+	stubsSuffix = getProp("PRODUCER_STUBS_CLASSIFIER") ?: "stubs"
 	if (getProp("EXTERNAL_CONTRACTS_ARTIFACT_ID")) {
 		logger.
 				lifecycle("Will use an artifact with contracts [${getProp("EXTERNAL_CONTRACTS_GROUP_ID")}:${getProp("EXTERNAL_CONTRACTS_ARTIFACT_ID")}]")

docs/src/main/asciidoc/verifier_setup.adoc

@@ -1084,6 +1084,7 @@ your running application, to the Artifact manager instance etc.
 - `PROJECT_GROUP` - your project's group id. Defaults to `com.example`
 - `PROJECT_VERSION` - your project's version. Defaults to `0.0.1-SNAPSHOT`
 - `PROJECT_NAME` - artifact id. Defaults to `example`
+- `PRODUCER_STUBS_CLASSIFIER` - archive classifier used for generated producer stubs, defaults to `stubs`.
 - `REPO_WITH_BINARIES_URL` - URL of your Artifact Manager. Defaults to `http://localhost:8081/artifactory/libs-release-local`
 which is the default URL of https://jfrog.com/artifactory/[Artifactory] running locally
 - `REPO_WITH_BINARIES_USERNAME` - (optional) username when the Artifact Manager is secured, defaults to `admin`.