Merge pull request #4569 from jedwards4b/machines_v3_fix

Add read of .cime/machine/config_machines.xml file if it exists.

Test suite: Hand testing with and without .cime/machine/config_machines.xml
Test baseline:
Test namelist changes:
Test status: bit for bit

Fixes #4568

User interface changes?:

Update gh-pages html (Y/N)?:

CIME/XML/machines.py

@@ -7,6 +7,7 @@
 from CIME.utils import convert_to_unknown_type, get_cime_config
 
 import socket
+from pathlib import Path
 
 logger = logging.getLogger(__name__)
 
@@ -317,15 +318,25 @@ def set_machine(self, machine, schema=None):
         if machine == "Query":
             return machine
         elif self.get_version() == 3:
-            machines_file = os.path.join(
-                self.machines_dir, machine, "config_machines.xml"
-            )
-            if os.path.isfile(machines_file):
+            machines_file = Path.home() / ".cime" / machine / "config_machines.xml"
+
+            if machines_file.exists():
                 GenericXML.read(
                     self,
                     machines_file,
                     schema=schema,
                 )
+            else:
+                machines_file = (
+                    Path(self.machines_dir) / machine / "config_machines.xml"
+                )
+
+                if machines_file.exists():
+                    GenericXML.read(
+                        self,
+                        machines_file,
+                        schema=schema,
+                    )
         self.machine_node = super(Machines, self).get_child(
             "machine",
             {"MACH": machine},