Paramiko's invoke_shell function does not take a command argument
even though the Bandit example implied that. It simply opens a stream
for communicating with a shell. Therefore, it should not be flagged
as part of the Bandit scan.

The current example of paramiko command injection does not properly
create an instance of the SSHClient before calling the functions
on the client. Instead it's calling the functions statically which
is not proper syntax.

This patches updates the plugin and example. Bandit, however, is
still functioning properly to detect the improper use of exec_command().

Fixes Issue #375

Signed-off-by: Eric Brown <browne@vmware.com>