Handle no roster file gracefully

Before this PR:

```
salt-ssh -c /home/sontek/src/salttest/smstack/etc/ '127.0.0.1' test.ping
[ERROR   ] An un-handled exception was caught by salt's global exception handler:
IOError: [Errno 2] No such file or directory: ''
Traceback (most recent call last):
  File "/home/sontek/venvs/salttest/bin/salt-ssh", line 9, in <module>
    load_entry_point('salt==2014.7.0-567-g6dae7e8', 'console_scripts', 'salt-ssh')()
  File "/home/sontek/src/salttest/salt/salt/scripts.py", line 252, in salt_ssh
    client.run()
  File "/home/sontek/src/salttest/salt/salt/cli/__init__.py", line 461, in run
    ssh = salt.client.ssh.SSH(self.config)
  File "/home/sontek/src/salttest/salt/salt/client/ssh/__init__.py", line 181, in __init__
    self.tgt_type)
  File "/home/sontek/src/salttest/salt/salt/roster/__init__.py", line 52, in targets
    targets.update(self.rosters[f_str](tgt, tgt_type))
  File "/home/sontek/src/salttest/salt/salt/roster/ansible.py", line 131, in targets
    imatcher = Inventory(tgt, tgt_type='glob', inventory_file=hosts)
  File "/home/sontek/src/salttest/salt/salt/roster/ansible.py", line 188, in __init__
    with salt.utils.fopen(inventory_file) as config:
  File "/home/sontek/src/salttest/salt/salt/utils/__init__.py", line 1110, in fopen
    fhandle = open(*args, **kwargs)
IOError: [Errno 2] No such file or directory: ''
```

and if you removed the ansible plugin:

```
salt-ssh -c /home/sontek/src/salttest/smstack/etc/ '*' test.ping
[ERROR   ] An un-handled exception was caught by salt's global exception handler:
SaltRenderError: Unable to render any roster.
Traceback (most recent call last):
  File "/home/sontek/venvs/salttest/bin/salt-ssh", line 9, in <module>
    load_entry_point('salt==2014.7.0-567-g6dae7e8', 'console_scripts', 'salt-ssh')()
  File "/home/sontek/src/salttest/salt/salt/scripts.py", line 252, in salt_ssh
    client.run()
  File "/home/sontek/src/salttest/salt/salt/cli/__init__.py", line 461, in run
    ssh = salt.client.ssh.SSH(self.config)
  File "/home/sontek/src/salttest/salt/salt/client/ssh/__init__.py", line 181, in __init__
    self.tgt_type)
  File "/home/sontek/src/salttest/salt/salt/roster/__init__.py", line 61, in targets
    raise salt.exceptions.SaltRenderError('Unable to render any roster.')
SaltRenderError: Unable to render any roster.
Traceback (most recent call last):
  File "/home/sontek/venvs/salttest/bin/salt-ssh", line 9, in <module>
    load_entry_point('salt==2014.7.0-567-g6dae7e8', 'console_scripts', 'salt-ssh')()
  File "/home/sontek/src/salttest/salt/salt/scripts.py", line 252, in salt_ssh
    client.run()
  File "/home/sontek/src/salttest/salt/salt/cli/__init__.py", line 461, in run
    ssh = salt.client.ssh.SSH(self.config)
  File "/home/sontek/src/salttest/salt/salt/client/ssh/__init__.py", line 181, in __init__
    self.tgt_type)
  File "/home/sontek/src/salttest/salt/salt/roster/__init__.py", line 61, in targets
    raise salt.exceptions.SaltRenderError('Unable to render any roster.')
salt.exceptions.SaltRenderError: Unable to render any roster.
```

Now it returns:
```
salt-ssh -c /home/sontek/src/salttest/smstack/etc/ '*' test.ping
Unable to render any roster.
```

and

```
salt-ssh -c /home/sontek/src/salttest/smstack/etc/ '127.0.0.1' test.ping
Permission denied for host 127.0.0.1, do you want to deploy the salt-ssh key? (password required):
[Y/n] n
127.0.0.1:
    ----------
    retcode:
        255
    stderr:
        Permission denied (publickey,password).
        
    stdout:
```