Make load order of local manifests deterministic

Local manifest files stored in the local_manifests folder are loaded
in alphabetical order, so it's easier to know in which order project
removals/additions/modifications will be applied.

If local_manifests.xml exists, it will be loaded before the files in
local_manifests.

Change-Id: Ia5c0349608f1823b4662cd6b340b99915bd973d5
diff --git a/manifest_xml.py b/manifest_xml.py
index a3e78fe..3198724 100644
--- a/manifest_xml.py
+++ b/manifest_xml.py
@@ -312,7 +312,7 @@
 
       local_dir = os.path.abspath(os.path.join(self.repodir, LOCAL_MANIFESTS_DIR_NAME))
       try:
-        for local_file in os.listdir(local_dir):
+        for local_file in sorted(os.listdir(local_dir)):
           if local_file.endswith('.xml'):
             try:
               nodes.append(self._ParseManifestXml(local_file, self.repodir))