[salt.cloud] key_filename is not always set for expanduser

[ERROR   ] There was a query error: 'NoneType' object has no attribute 'startswith'
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/salt/cloud/cli.py", line 381, in run
    ret = mapper.run_map(dmap)
  File "/usr/lib/python2.7/site-packages/salt/cloud/__init__.py", line 1692, in run_map
    profile, local_master=local_master
  File "/usr/lib/python2.7/site-packages/salt/cloud/__init__.py", line 905, in create
    output = self.clouds[func](vm_)
  File "/usr/lib/python2.7/site-packages/salt/cloud/clouds/openstack.py", line 329, in create
    key_filename = os.path.expanduser(key_filename)
  File "/usr/lib64/python2.7/posixpath.py", line 261, in expanduser
    if not path.startswith('~'):
AttributeError: 'NoneType' object has no attribute 'startswith'