##### SUMMARY

SyntaxWarning over using is for comparison of literals. The fix is simple and is a good beginner issue.

```
contrib/inventory/cobbler.py:218: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if this_dns_name is not None and this_dns_name is not "":
```

##### ISSUE TYPE

- Bug Report

##### COMPONENT NAME

cobbler

##### ANSIBLE VERSION

devel branch
if not self.conn:
            self.conn = xmlrpclib.Server(self.cobbler_host, allow_none=True)
            self.token = None
            if self.cobbler_username is not None:
                self.token = self.conn.login(self.cobbler_username, self.cobbler_password)
    def is_cache_valid(self):