Implement instancewrapper.instance_settings()

[espenak]

The motivation is clearly stated in #2. We should be able to use:

with instancewrapper.instance_settings():
    print_uname()

As a shortcut for:

ssh_uri = instancewrapper.get_ssh_uri()
key_filename = instancewrapper.get_ssh_key_filename()
with settings(key_filename=key_filename, host_string=ssh_uri):
    # Run tasks here just as if you invoked ``awsfab -E myname <task>``
    print_uname()