Rename ArgumentValidatingCommand as SSHCommand

[ncouture]

All commands can and should be validating arguments. Commands requiring no argument should be considered "validly typed" if their arguments are None.

With this in mind and as ArgumentValidatingCommand is a subclass of SSHCommand and should be able to provide all of its superclass' functionality, it would simplify MockSSH from a usage perspective to consolidate these classes functionalities under the same one.

With this fix:

1. ArgumentValidatingCommand is renamed to SSHCommand
2. SSHCommand is renamed to _SSHCommand
3. ArgumentValidatingCommand is removed from all
4. Examples (examples/mock_cisco.py and examples/mock_F5.py) are updated to reflect this change
5. The hy DSL is updated to reflect this change

[ncouture]

The ArgumentValidatingCommand does not support unknown positional arguments, this is a problem in this use case (see examples/mock_cisco.py):

username john password jd03

In order to replace SSHCommand with ArgumentValidatingCommand we would need to provide that functionality.

Here is an example of what it should support:

command_username = MockSSH.SSHCommand('username', 
                                     [username_command_success], 
                                     [username_command_failure], 
                                     *["<username>", "password", "<password>"])

Where "" and "" are substitutes for anything; wildcards.