Fix connection plugin to work with Ansible 2.6.0
Fix connection plugin to work with Ansible 2.6.0

The connection plugin needs to follow the upstream connection/ssh
plugin's docstring, otherwise this causes failures for undefined options
sftp_executable and scp_executable. This PR adds those docstrings to
mirror upstream.

This follows on from a patch in upstream ansible:
https://github.com/ansible/ansible/pull/36648/files

Change-Id: Ia2a2f2238bb60710c7efe76e2cd9f86493833a2d

openstack/openstack-ansible-plugins,openstack/openstack-ansible-plugins