libssh: Use more verbose logging

The `INFO` log level is usually not enough to get useful information from the libssh regarding the low-level issues that we would like to be able to resolve. This can be made even more fine grained, but I do not think less verbose logs would be much useful anyway.

Related is the issue in pylibssh, which maps the DEBUG verbosity to something that is not debug in libssh itself:

ansible/pylibssh#597

plugins/connection/libssh.py

@@ -376,7 +376,7 @@ def _connect_uncached(self):
         self.ssh = Session()
 
         if display.verbosity > 3:
-            self.ssh.set_log_level(logging.INFO)
+            self.ssh.set_log_level(logging.DEBUG)
 
         self.keyfile = os.path.expanduser("~/.ssh/known_hosts")