Implement Report persistency

Now Report properties are stored in non-volatile memory. It allows
to restore Report after system restart. Persistency of a report is
controlled by Persistency property in Report interface.

Tested:
 - Passed unit tests
 - Verified that report is stored in /var/lib/telemetry dir
 - Verified that report is restored from storage after telemetry
   service start

Signed-off-by: Wludzik, Jozef <jozef.wludzik@intel.com>
Change-Id: Iccfe21603eecffc4e174a4403f699b03de320db9
diff --git a/tests/src/mocks/metric_mock.hpp b/tests/src/mocks/metric_mock.hpp
index 6150455..d3d9e64 100644
--- a/tests/src/mocks/metric_mock.hpp
+++ b/tests/src/mocks/metric_mock.hpp
@@ -17,4 +17,5 @@
 
     MOCK_METHOD(const std::vector<MetricValue>&, getReadings, (),
                 (const, override));
+    MOCK_METHOD(nlohmann::json, to_json, (), (const, override));
 };