Fix TypeError with newer version of lshw

The issue with json output in lshw was fixed in version B.02.19
This patch makes the memory calculation compatible with that
version and later versions that are included in recent distributions
(e.g. Ubuntu 20.04, Fedora 31)

Change-Id: Id5a30028b139c51cae6232cac73a50b917fea233
Story: 2007588
Task: 39527

ironic_python_agent/hardware.py

@@ -1003,13 +1003,14 @@ class GenericHardwareManager(HardwareManager):
             total = None
             LOG.exception(("Cannot fetch total memory size using psutil "
                            "version %s"), psutil.version_info[0])
-        sys_dict = None
         try:
             sys_dict = _get_system_lshw_dict()
         except (processutils.ProcessExecutionError, OSError, ValueError) as e:
             LOG.warning('Could not get real physical RAM from lshw: %s', e)
             physical = None
         else:
+            if isinstance(sys_dict, str):
+                sys_dict = json.loads(sys_dict)
             physical = _calc_memory(sys_dict)
 
             if not physical: