cron: change identifier default value to False

this allows the user to set it explicitly to None if they want to
manage a cronjob with no identifier. realistically, this is probably
a rare usecase.

salt/states/cron.py

@@ -216,7 +216,7 @@ def present(name,
             month='*',
             dayweek='*',
             comment=None,
-            identifier=None):
+            identifier=False):
     '''
     Verifies that the specified cron job is present for the specified user.
     For more advanced information about what exactly can be set in the cron
@@ -256,7 +256,7 @@ def present(name,
         edits. This defaults to the state id
     '''
     name = ' '.join(name.strip().split())
-    if not identifier:
+    if identifier is False:
         identifier = name
     ret = {'changes': {},
            'comment': '',
@@ -312,7 +312,7 @@ def present(name,
 
 def absent(name,
            user='root',
-           identifier=None,
+           identifier=False,
            **kwargs):
     '''
     Verifies that the specified cron job is absent for the specified user; only
@@ -334,7 +334,7 @@ def absent(name,
     ###       of unsupported arguments will result in a traceback.
 
     name = ' '.join(name.strip().split())
-    if not identifier:
+    if identifier is False:
         identifier = name
     ret = {'name': name,
            'result': True,