Feature target metadata (#757)

* Defined the model for target matadata and the corresponding repository layer/management
* Added target metadata quotas incl enforcement
* Extended Target Mgmt REST API to allow for metadata CRUD operations
* Added migration scripts for each database
* Added back reference to target metadata in JpaTarget
* Added tests for target management, Mgmt REST API, target metadata RSQL, and REST documentation
* Updated asciidocs for target rest documentation
* Fix Allure imports and annotations
* Fix review findings

Signed-off-by: Bogdan Bondar <Bogdan.Bondar@bosch-si.com>
Signed-off-by: Stefan Behl <stefan.behl@bosch-si.com>

hawkbit-core/src/main/java/org/eclipse/hawkbit/repository/TargetMetadataFields.java

@@ -0,0 +1,36 @@
+/**
+ * Copyright (c) 2018 Bosch Software Innovations GmbH and others.
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/legal/epl-v10.html
+ */
+package org.eclipse.hawkbit.repository;
+
+/**
+ * Sort fields for TargetMetadata.
+ *
+ */
+public enum TargetMetadataFields implements FieldNameProvider {
+
+    /**
+     * The value field.
+     */
+    VALUE("value"),
+    /**
+     * The key field.
+     */
+    KEY("key");
+
+    private final String fieldName;
+
+    TargetMetadataFields(final String fieldName) {
+        this.fieldName = fieldName;
+    }
+
+    @Override
+    public String getFieldName() {
+        return fieldName;
+    }
+}