Add a connection decorator to the ProxyKmipClient
Add a connection decorator to the ProxyKmipClient

This change simplifies how the ProxyKmipClient enforces the open
connection check on operation method calls. Each operation method
is now wrapped with an is_connected decorator which handles the
connection check. All old in-method connection checks have been
removed.

OpenKMIP/PyKMIP,OpenKMIP/PyKMIP