Fix overrides of plugin repository

The correct way to override the plugins repo is through the pluginManagement
section in the gradle settings file.  Also make use of the gradle.properties
file to initialize repositoryUrl and also publishUrl so we don't have to mess
around with finding and assigning them in the main gradle file.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=233778270

gradle/build.gradle

@@ -1,18 +1,4 @@
 buildscript {
-  ext.repositoryUrl = project.findProperty('repositoryUrl')
-  ext.publishUrl = project.findProperty('publishUrl')
-  repositories {
-    if (repositoryUrl == null) {
-      println "Using Maven central..."
-      mavenCentral()
-    } else {
-      maven {
-        println "Using GCS Maven repo..."
-        url repositoryUrl
-      }
-    }
-  }
-
   // Lock buildscript dependencies.
   configurations.classpath {
     resolutionStrategy.activateDependencyLocking()
@@ -71,7 +57,7 @@ task stage {
 }
 
 
-if (publishUrl != null) {
+if (publishUrl) {
   publishing {
     repositories {
       maven {
@@ -129,12 +115,12 @@ allprojects {
   if (project.name == 'services') return
 
   repositories {
-    if (rootProject.repositoryUrl == null) {
-      mavenCentral()
-    } else {
+    if (rootProject.repositoryUrl) {
       maven {
         url rootProject.repositoryUrl
       }
+    } else {
+      mavenCentral()
     }
   }
 

gradle/gradle.properties

@@ -0,0 +1,2 @@
+repositoryUrl=
+publishUrl=

gradle/settings.gradle

@@ -1,3 +1,16 @@
+if (repositoryUrl) {
+  println "Using repository $repositoryUrl for plugins"
+  pluginManagement {
+    repositories {
+      maven {
+        url repositoryUrl
+      }
+    }
+  }
+} else {
+  println "Using default repository for plugins"
+}
+
 rootProject.name = 'nomulus'
 
 include 'core'