build: Don't override bintray artifact during uploading by default

This patch disables overriding the gradle-plugin artifact if
such artifact is already uploaded to bintray. One may force
overriding using 'override' project property:
./gradlew bintrayUpload -Poverride

tools/gradle-plugin/build.gradle

@@ -96,8 +96,8 @@ bintray {
             name = '0.1'
             desc = 'Kotlin Native Gradle plugin 0.1'
         }
-        publish = true
+        publish = true // project.hasProperty("publish")
-        override = true
+        override = project.hasProperty("override")
     }
     publications = ['gradlePlugin']
 }