Fixed errors in FreeBSD user manipulations.

system/user

@@ -568,7 +568,7 @@ class FreeBsdUser(User):
             cmd.append(self.home)
 
         if self.group is not None:
-            if not user.group_exists(self.group):
+            if not self.group_exists(self.group):
                 self.module.fail_json(msg="Group %s does not exist" % self.group)
             cmd.append('-g')
             cmd.append(self.group)
@@ -631,7 +631,7 @@ class FreeBsdUser(User):
             cmd.append(self.home)
 
         if self.group is not None:
-            if not user.group_exists(self.group):
+            if not self.group_exists(self.group):
                 self.module.fail_json(msg="Group %s does not exist" % self.group)
             ginfo = self.group_info(self.group)
             if info[3] != ginfo[2]: