[mle] let the command partitionid return the current partition id (#5872)

If user doesn't set the Partition Id using the command
'leaderpartitionid xxx', the current cli command 'leaderpartitionid'
always return 0. This commit renames the command 'leaderpartitionid'
to partitionid and let the command partitionid returns the current
leader Partition Id and add command partitionid preferred to set or
get preferred Partition Id.

tests/scripts/thread-cert/Cert_5_5_03_SplitMergeChildren.py

@@ -114,7 +114,7 @@ class Cert_5_5_3_SplitMergeChildren(thread_cert.TestCase):
 
         self.nodes[LEADER].reset()
         self._setUpLeader()
-        self.nodes[ROUTER2].set_partition_id(0xffffffff)
+        self.nodes[ROUTER2].set_preferred_partition_id(0xffffffff)
 
         self.simulator.go(140)
 

tests/scripts/thread-cert/node.py

@@ -939,12 +939,12 @@ class NodeImpl:
         self.send_command(cmd)
         self._expect('Done')
 
-    def get_partition_id(self):
-        self.send_command('leaderpartitionid')
+    def get_preferred_partition_id(self):
+        self.send_command('partitionid preferred')
         return self._expect_result(r'\d+')
 
-    def set_partition_id(self, partition_id):
-        cmd = 'leaderpartitionid %d' % partition_id
+    def set_preferred_partition_id(self, partition_id):
+        cmd = 'partitionid preferred %d' % partition_id
         self.send_command(cmd)
         self._expect('Done')
 

tests/scripts/thread-cert/thread_cert.py

@@ -161,7 +161,7 @@ class TestCase(NcpSupportMixin, unittest.TestCase):
             self.nodes[i].set_mode(params['mode'])
 
             if 'partition_id' in params:
-                self.nodes[i].set_partition_id(params['partition_id'])
+                self.nodes[i].set_preferred_partition_id(params['partition_id'])
             if 'channel' in params:
                 self.nodes[i].set_channel(params['channel'])
             if 'masterkey' in params: