[ONOS-6410] flexible configuration datastore specifier.

- added completer to netconf-config-get

Change-Id: I7cc88637bd51d9f4bea7d906346ffacfbd8706a6
diff --git a/protocols/netconf/ctl/src/main/java/org/onosproject/netconf/cli/impl/completers/DatastoreIdCompleter.java b/protocols/netconf/ctl/src/main/java/org/onosproject/netconf/cli/impl/completers/DatastoreIdCompleter.java
new file mode 100644
index 0000000..3981bed
--- /dev/null
+++ b/protocols/netconf/ctl/src/main/java/org/onosproject/netconf/cli/impl/completers/DatastoreIdCompleter.java
@@ -0,0 +1,52 @@
+/*
+ * Copyright 2017-present Open Networking Laboratory
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.onosproject.netconf.cli.impl.completers;
+
+import java.lang.reflect.Modifier;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Objects;
+import java.util.stream.Collectors;
+
+import org.onosproject.cli.AbstractChoicesCompleter;
+import org.onosproject.netconf.DatastoreId;
+
+/**
+ * Completer for predefined {@link DatastoreId}.
+ *
+ */
+public class DatastoreIdCompleter extends AbstractChoicesCompleter {
+
+    @Override
+    protected List<String> choices() {
+        // TODO: Ideally candidates should be chosen based on Device capability
+        return Arrays.asList(DatastoreId.class.getFields())
+                .stream()
+                .filter(f -> f.getType() == DatastoreId.class)
+                .filter(f -> Modifier.isStatic(f.getModifiers()))
+                .map(f -> {
+                    try {
+                        return (DatastoreId) f.get(null);
+                    } catch (IllegalArgumentException | IllegalAccessException e) {
+                        return null;
+                    }
+                })
+                .filter(Objects::nonNull)
+                .map(DatastoreId::id)
+                .collect(Collectors.toList());
+    }
+
+}