Arch distro fails to write network config with empty dns-nameservers

Bug Description

In distros/arch.py, the network configuration is created using

'DNS': str(tuple(info.get('dns-nameservers'))).replace(',', '')

However, when dns-nameservers is None, this causes both cloud-init and cloud-init-local to fail with

failed run of stage init-local
------------------------------------------------------------
Traceback (most recent call last):
File "/usr/lib/python2.7/site-packages/cloudinit/cmd/main.py", line 513, in status_wrap
ret = functor(name, args)
File "/usr/lib/python2.7/site-packages/cloudinit/cmd/main.py", line 254, in main_init
init.apply_network_config(bring_up=not args.local)
File "/usr/lib/python2.7/site-packages/cloudinit/stages.py", line 641, in apply_network
return self.distro.apply_network_config(netcfg, bring_up=bring_up)
File "/usr/lib/python2.7/site-packages/cloudinit/distros/__init__.py", line 154, in app
netconfig, bring_up=bring_up)
File "/usr/lib/python2.7/site-packages/cloudinit/distros/__init__.py", line 143, in _ap
return self.apply_network(contents, bring_up=bring_up)
File "/usr/lib/python2.7/site-packages/cloudinit/distros/__init__.py", line 125, in app
dev_names = self._write_network(settings)
File "/usr/lib/python2.7/site-packages/cloudinit/distros/arch.py", line 67, in _write_n
'DNS': str(tuple(info.get('dns-nameservers'))).replace(',', '')
TypeError: 'NoneType' object is not iterable
------------------------------------------------------------

The fix proposed in https://bbs.archlinux.org/viewtopic.php?pid=1662566#p1662566 seems to work for me, namely replacing the line with

'DNS': str(tuple(info.get('dns-nameservers'))).replace(',', '') if info.get('dns-nameservers') != None else None

A more concise patch borrowed from https://github.com/hartwork/image-bootstrap/commit/6f2a6709302c17f8d4775df479e7b636bbb8f008

This bug is believed to be fixed in cloud-init in 17.1. If this is still a problem for you, please make a comment and set the state back to New

Thank you.

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/2801