[FIX] unnecessary rebuilding of assets

The checksum computed to verify the version of the assets includes a list of
files as JSON. This JSON is generated from a list of dictionaries.
When using Python <3.6, the ordering of the dictionaries is not guaranteed which
lead to unnecessary rebuild of assets. Particularly when starting Odoo on more
than one host, the current implementation hits this issue:

* server 1 computes the checksum with an ordering and generates the assets
* server 2 computes the checksum with a different ordering and generates the assets
* server 1 computes the checksum with a different ordering and generates the assets
* And so on...

Which obviously lead to disastrous performances.

odoo/addons/base/ir/ir_qweb/assetsbundle.py

@@ -152,7 +152,7 @@ class AssetsBundle(object):
         Not really a full checksum.
         We compute a SHA1 on the rendered bundle + max linked files last_modified date
         """
-        check = u"%s%s%s" % (json.dumps(self.files), u",".join(self.remains), self.last_modified)
+        check = u"%s%s%s" % (json.dumps(self.files, sort_keys=True), u",".join(self.remains), self.last_modified)
         return hashlib.sha1(check.encode('utf-8')).hexdigest()
 
     def clean_attachments(self, type):