Set the DiskAccountant log as warning level

This is to help operator to find DiskAccountant related issue.
The previous info level does not reflect the real log severity.

Change-Id: I222d460a5e7630901f283f2a1109e37062f963a2

zuul/executor/server.py

@@ -143,7 +143,7 @@ class DiskAccountant(object):
                     continue
                 size = int(size)
                 if size > self.limit:
-                    self.log.info(
+                    self.log.warning(
                         "{job} is using {size}MB (limit={limit})"
                         .format(size=size, job=dirname, limit=self.limit))
                     self.func(dirname)