Config dict arguments.url must be a list

For memcache, arguments.url is assumed to be a list, this means
that the underlying memcache library could not handle a case where
a string (comma separated) was passed in. The idea is to force a
split on ',' in the special case of backend_argument=url:...
so that a list is passed to the backend.

Change-Id: I65afb46455423d7f96e17c471330c8d74dc17cac
Closes-Bug: #1743036

oslo_cache/core.py

@@ -120,6 +120,8 @@ def _build_cache_config(conf):
             continue
 
         arg_key = '.'.join([prefix, 'arguments', argname])
+        if argname == 'url':
+            argvalue = argvalue.split(',')
         conf_dict[arg_key] = argvalue
 
         _LOG.debug('Oslo Cache Config: %s', conf_dict)

releasenotes/notes/bug-1743036-320ed918d5fb4325.yaml

@@ -0,0 +1,10 @@
+---
+other:
+  - >
+    [`bug 1743036 <https://bugs.launchpad.net/oslo.cache+bug/1743036>`_]
+    The `backend_argument` value(s) for `url` when configuring memcache did not
+    properly handle multiple servers. This is because the URL was passed as
+    a string (comma delimited) instead of a list to the memcache
+    library/client. The `url` argument is now special cased and will split
+    the string on a comma so that it mirrors the behavior of the ListOpt used
+    by `memcache_servers` option.