[build] Allow selection of a specific Netty version

By using the Gradle property `forceNettyVersion`, one can select an
arbitrary version of Netty, eg. to run tests with the latest snapshots
in CI in parallel to the current release.

Snapshot repository for Netty is also added in case the current reactor
version is a snapshot itself.

build.gradle

@@ -60,8 +60,17 @@ ext {
   testAddonVersion = reactorCoreVersion
   assertJVersion = '3.6.1'
 
+  // Netty
+  nettyDefaultVersion = '4.1.43.Final'
+  if (!project.hasProperty("forceNettyVersion")) {
+	nettyVersion = nettyDefaultVersion
+  }
+  else {
+	nettyVersion = forceNettyVersion
+	println "Netty version defined from command line: ${forceNettyVersion}"
+  }
+
   // Libraries
-  nettyVersion = '4.1.43.Final'
   jacksonDatabindVersion = '2.5.1'
 
   // Testing
@@ -203,6 +212,7 @@ configure(rootProject) { project ->
 		println 'Bamboo CI detected, avoiding use of mavenLocal()'
 	  }
 	  maven { url 'https://repo.spring.io/libs-snapshot' }
+	  maven { url 'https://oss.sonatype.org/content/repositories/snapshots' }
 	}
 	maven { url 'https://repo.spring.io/libs-milestone' }
 	maven { url 'https://repo.spring.io/libs-release' }