Apply Artifactory Gradle plugin

Instead of relying on the CI server to apply and configure this plugin,
this commit does it directly in the Spring Framework build.
This allows us to take full control over which projects are published
and how.

See gh-23282

integration-tests/integration-tests.gradle

@@ -20,8 +20,3 @@ dependencies {
 	testCompile("org.hsqldb:hsqldb:${hsqldbVersion}")
 	testCompile("org.hibernate:hibernate-core:5.1.17.Final")
 }
-
-// Avoid publishing JAR to the artifact repository
-if (pluginManager.hasPlugin("com.jfrog.artifactory")) {
-	artifactoryPublish.skip = true
-}