Add support for libssh connection type

*  Add libssh connection type that uses
   pylibssh library (https://github.com/ansible/pylibssh)
   which is Python extension for libssh (https://www.libssh.org/)
*  Refactor network_cli connection plugin to work with both
   libssh and paramiko_ssh connetion type under the hood.
   The default connection type is set to libssh and can be
   configured using `ssh_transport_type` network_cli
   plugin configuration.

lib/ansible/plugins/cliconf/__init__.py

@@ -340,7 +340,7 @@ def copy_file(self, source=None, destination=None, proto='scp', timeout=30):
                         remote host before triggering timeout exception
         :return: None
         """
-        ssh = self._connection.paramiko_conn._connect_uncached()
+        ssh = self._connection.transport_conn._connect_uncached()
         if proto == 'scp':
             if not HAS_SCP:
                 raise AnsibleError("Required library scp is not installed.  Please install it using `pip install scp`")
@@ -361,7 +361,7 @@ def get_file(self, source=None, destination=None, proto='scp', timeout=30):
         :return: None
         """
         """Fetch file over scp/sftp from remote device"""
-        ssh = self._connection.paramiko_conn._connect_uncached()
+        ssh = self._connection.transport_conn._connect_uncached()
         if proto == 'scp':
             if not HAS_SCP:
                 raise AnsibleError("Required library scp is not installed.  Please install it using `pip install scp`")