fix validation error

[pwyliu]

In 4.1.0 the UseDNS fragment was added to the sshd template. This causes a validation error when sftp_enabled is true, because it's below Match Group sftponly and sshd interprets this line to be inside the Match block:

...
TASK [dev-sec.ssh-hardening : create sshd_config and set permissions to root/600] ***
fatal: [default]: FAILED! => {"changed": true, "exit_status": 255, "failed": true, "msg": "failed to validate", "stderr": "/home/vagrant/.ansible/tmp/ansible-tmp-1494819613.2-15252609610528/source line 164: Directive 'UseDNS' is not allowed within a Match block\r\n", 
...

This PR moves the UseDNS fragment up into the network section to resolve this.

[rndmh3ro]

Thanks for the fix!