[2.1] mDNS label contains disallowed characters

[Gavin Panella]

At home I have a device on my network with the name "Living room". This is escaped by Avahi to "Living\032room", but this is rejected by MAAS.

1. This name ought to be accepted by MAAS.

2. Properly invalid names should not cause a crash like this.

Traceback (most recent call last):
  File "/usr/lib/python3.5/threading.py", line 862, in run
    self._target(*self._args, **self._kwargs)
  File ".../provisioningserver/utils/twisted.py", line 790, in worker
    return target()
  File ".../twisted/_threads/_threadworker.py", line 46, in work
    task()
  File ".../twisted/_threads/_team.py", line 190, in doWork
    task()
--- <exception caught here> ---
  File ".../twisted/python/threadpool.py", line 246, in inContext
    result = inContext.theWork()
  File ".../twisted/python/threadpool.py", line 262, in <lambda>
    inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
  File ".../twisted/python/context.py", line 118, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File ".../twisted/python/context.py", line 81, in callWithContext
    return func(*args,**kw)
  File ".../provisioningserver/utils/twisted.py", line 823, in
  callInContext
    return func(*args, **kwargs)
  File ".../maasserver/utils/orm.py", line 603, in
  call_within_transaction
    return func_outside_txn(*args, **kwargs)
  File ".../maasserver/utils/orm.py", line 422, in retrier
    return func(*args, **kwargs)
  File "/usr/lib/python3.5/contextlib.py", line 30, in inner
    return func(*args, **kwds)
  File ".../maasserver/regiondservices/networks_monitoring.py", line 60,
  in recordMDNSEntriesIntoDatabase
    region_controller.report_mdns_entries(mdns)
  File ".../maasserver/models/node.py", line 4041, in
  report_mdns_entries
    interface.update_mdns_entry(entry)
  File ".../maasserver/models/interface.py", line 1225, in
  update_mdns_entry
    interface=self, ip=ip, hostname=hostname)
  File ".../django/db/models/manager.py", line 127, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File ".../django/db/models/query.py", line 348, in create
    obj.save(force_insert=True, using=self.db)
  File ".../maasserver/models/cleansave.py", line 28, in save
    self.full_clean()
  File ".../django/db/models/base.py", line 1171, in full_clean
    raise ValidationError(errors)
django.core.exceptions.ValidationError: {'hostname': ["Label contains
disallowed characters: 'Living\\\\032room'."]}