[ONOS-6410] flexible configuration datastore specifier.

- added completer to netconf-config-get

Change-Id: I7cc88637bd51d9f4bea7d906346ffacfbd8706a6
diff --git a/protocols/netconf/api/src/main/java/org/onosproject/netconf/DatastoreId.java b/protocols/netconf/api/src/main/java/org/onosproject/netconf/DatastoreId.java
new file mode 100644
index 0000000..b0df547
--- /dev/null
+++ b/protocols/netconf/api/src/main/java/org/onosproject/netconf/DatastoreId.java
@@ -0,0 +1,79 @@
+/*
+ * 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;
+
+import static com.google.common.base.Preconditions.checkArgument;
+
+import org.onlab.util.Identifier;
+
+/**
+ * Identifier object to specify datastore.
+ */
+public class DatastoreId extends Identifier<String> {
+
+    /**
+     * A configuration datastore holding
+     * the complete configuration currently active on the device.  The
+     * running configuration datastore always exists.
+     */
+    public static final DatastoreId RUNNING = datastore("running");
+
+    /**
+     * A configuration datastore that
+     * can be manipulated without impacting the device's current
+     * configuration and that can be committed to the running
+     * configuration datastore.  Not all devices support a candidate
+     * configuration datastore.
+     */
+    public static final DatastoreId CANDIDATE = datastore("candidate");
+
+    /**
+     * The configuration datastore
+     * holding the configuration loaded by the device when it boots.
+     * Only present on devices that separate the startup configuration
+     * datastore from the running configuration datastore.
+     */
+    public static final DatastoreId STARTUP = datastore("startup");
+
+    /**
+     * Returns datastore identifier object.
+     *
+     * @param name of the datastore
+     * @return identifier
+     */
+    public static DatastoreId datastore(String name) {
+        return new DatastoreId(name);
+    }
+
+    DatastoreId(String name) {
+        super(name);
+        checkArgument(!name.isEmpty());
+    }
+
+    /**
+     * Returns datastore name as XML tag.
+     * @return xml tag
+     */
+    public String asXml() {
+        return "<" + id() + "/>";
+    }
+
+    @Override
+    public final String toString() {
+        return id();
+    }
+
+}