Partially convert p4 to use attribute group for 'mode'

The check if self.mode is in possible_modes can be changed but there's
also a check if it's a renderable. That's different from everything else
so I am skipping it until the history is better understood.

master/buildbot/steps/source/p4.py

@@ -122,18 +122,15 @@ def checkInstall(p4Installed):
         if self.use_tickets and self.p4passwd:
             d.addCallback(self._acquireTicket)
 
-        if self.mode == 'full':
-            d.addCallback(self.full)
-        elif self.mode == 'incremental':
-            d.addCallback(self.incremental)
+        d.addCallback(self._getAttrGroupMember('mode', self.mode))
 
         d.addCallback(self.parseGotRevision)
         d.addCallback(self.finish)
         d.addErrback(self.failed)
         return d
 
     @defer.inlineCallbacks
-    def full(self, _):
+    def mode_full(self, _):
         if debug_logging:
             log.msg("P4:full()..")
 
@@ -160,7 +157,7 @@ def full(self, _):
             log.msg("P4: full() sync done.")
 
     @defer.inlineCallbacks
-    def incremental(self, _):
+    def mode_incremental(self, _):
         if debug_logging:
             log.msg("P4:incremental()")