Deprecate -d switch for running in foreground

Until the -f switch was introduced the only way of running in
foreground was the -d switch which also implied debug logging. We now
have the -f switch so we can decouple that in future. Thus depreciate
the -d switch for the sake of running in forground.

Change-Id: Ic66ba1a28b8e2837309df49fc0ff28d2495fc229

releasenotes/notes/foreground-c210261012f433fd.yaml

@@ -0,0 +1,6 @@
+---
+deprecations:
+  - |
+    Running the zuul processes with the -d switch to enforce running in
+    foreground is now deprecated. Switch to use -f instead for this purpose.
+    The -d switch will change to just enable debug logging in the future.

zuul/cmd/__init__.py

@@ -174,7 +174,11 @@ class ZuulDaemonApp(ZuulApp, metaclass=abc.ABCMeta):
     def createParser(self):
         parser = super(ZuulDaemonApp, self).createParser()
         parser.add_argument('-d', dest='debug', action='store_true',
-                            help='run in foreground with debug log')
+                            help='run in foreground with debug log. Note '
+                                 'that in future this will be changed to only '
+                                 'request debug logging. If you want to keep '
+                                 'running the process in the foreground '
+                                 'migrate/add the -f switch.')
         parser.add_argument('-f', dest='foreground', action='store_true',
                             help='run in foreground with info log')
         return parser