Fix an issue where git-pull fails with AttributeError

As part of 94f3b9bfab69feefa37bb006bf75f4a1b8acb19f the code was changed to support dynamically adding localhost to the inventory. This change introduced an crash when run via ansible-pull

```
Starting ansible-pull at 2014-01-20 23:09:57
Traceback (most recent call last):
  File "/tmp/ansible/bin/ansible", line 157, in <module>
    (runner, results) = cli.run(options, args)
  File "/tmp/ansible/bin/ansible", line 82, in run
    hosts = inventory_manager.list_hosts(pattern)
  File "/tmp/ansible/lib/ansible/inventory/__init__.py", line 372, in list_hosts
    result = [ h.name for h in self.get_hosts(pattern) ]
  File "/tmp/ansible/lib/ansible/inventory/__init__.py", line 136, in get_hosts
    subset = self._get_hosts(self._subset)
  File "/tmp/ansible/lib/ansible/inventory/__init__.py", line 177, in _get_hosts
    that = self.__get_hosts(p)
  File "/tmp/ansible/lib/ansible/inventory/__init__.py", line 198, in __get_hosts
    hpat = self._hosts_in_unenumerated_pattern(name)
  File "/tmp/ansible/lib/ansible/inventory/__init__.py", line 275, in _hosts_in_unenumerated_pattern
    ungrouped.add_host(new_host)
AttributeError: 'NoneType' object has no attribute 'add_host'
```

The root cause is there is no group for the host to be added to. I fixed this case by creating the ungrouped group when it doesn't exist and then adding the host to the newly added group. This fixes the regression for me.
Merge pull request #5697 from kormoc/fix_missing_localhost_group

Fix an issue where ansbile-pull fails with AttributeError