Build: Remove public license key and inject it during release builds (elastic/x-pack-elasticsearch#3916)

This commit moves the dev key into core and renames to make it clear it
is for snapshots, and makes the production key a required parameter of
release builds.

Original commit: elastic/x-pack-elasticsearch@ea299bd5a2

plugin/core/build.gradle

@@ -58,10 +58,16 @@ processResources {
     MavenFilteringHack.filter(it, expansions)
   }
   boolean snapshot = "true".equals(System.getProperty("build.snapshot", "true"))
-  if (snapshot) {
-    from '../keys/dev/public.key'
+  String licenseKey = System.getProperty("license.key")
+  if (licenseKey != null) {
+    println "Using provided license key from ${licenseKey}"
+  } else if (snapshot) {
+    licenseKey = 'snapshot.key'
   } else {
-    from '../keys/prod/public.key'
+    throw new IllegalArgumentException('Property license.key must be set for release build')
+  }
+  from(licenseKey) {
+    rename { String filename -> 'public.key' }
   }
 }
 

plugin/keys/prod/public.key

@@ -1,3 +0,0 @@
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