config: add support for environment variable injection

Motivation:
in container world it's common to use environment variables to pass the
configuration parameters. Currently dCache doesn't support such injects,
which makes container-based deployments more complicated.

Modification:
Update ConfigurationProperties to resolve environment variables when
reading the property (layout, conf) files. The syntax is:

dcache.net.wan.port.min=${env.DCACHE_WAN_PORT_MIN}
dcache.net.wan.port.max=${env.DCACHE_WAN_PORT_MAX}

Result:

dcache properties can be controlled by env variables, if desired.

Acked-by: Albert Rossi
Target: master
Require-book: yes
Require-notes: yes

modules/dcache/src/main/java/org/dcache/util/configuration/ConfigurationProperties.java

@@ -247,14 +247,28 @@ public synchronized Object put(Object rawKey, Object value) {
             _prefixes.add(key.getPropertyName() + PREFIX_SEPARATOR);
         }
 
-        checkIsAllowed(key, (String) value);
+        // the reset of the code expects `value` to be an instance of String
+        assert value instanceof String;
+
+        String stringValue = (String) value;
+        // REVISIT: do we need to resolve it here or on get ?
+        if (stringValue.startsWith("${env.") && stringValue.endsWith("}")) {
+            String envKey = stringValue.substring(6, stringValue.length() - 1);
+            stringValue = System.getenv(envKey);
+            if (stringValue == null) {
+                _problemConsumer.error("Environment variable " + envKey + " for property " + rawKey + " undefined.");
+                return null;
+            }
+        }
+
+        checkIsAllowed(key, stringValue);
 
         if (key.hasAnnotations()) {
             putAnnotatedKey(key);
         }
 
         return key.hasAnyOf(OBSOLETE_FORBIDDEN) ? null
-              : super.put(name, canonicalizeValue(name, value));
+              : super.put(name, canonicalizeValue(name, stringValue));
     }
 
     private String canonicalizeValue(String key, Object value) {

modules/dcache/src/test/java/org/dcache/util/configuration/ConfigurationPropertiesTests.java

@@ -399,6 +399,20 @@ public void testGetReplacementExpandingWithSpace() {
         assertEquals(valueWithSpace, _standardProperties.getValue(EXPANDING_PROPERTY_NAME));
     }
 
+    @Test
+    public void testGetReplacementExpandingEnv() {
+        //  we assume that this SHELL env variable always set
+        _standardProperties.setProperty(SIMPLE_PROPERTY_NAME, propertyReference("env.SHELL"));
+        _standardProperties.getProperty(SIMPLE_PROPERTY_NAME);
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testGetReplacementMissingEnv() {
+        //  we assume that this env variable actually never exists
+        _standardProperties.setProperty(SIMPLE_PROPERTY_NAME, propertyReference("env.NO_NO_NO"));
+        _standardProperties.getProperty(SIMPLE_PROPERTY_NAME);
+    }
+
     @Test
     public void testTwoDeepExpansion() {
         String expanding1Value = propertyReference(SIMPLE_PROPERTY_NAME);