cli_netlogon: Rename "netlogon_creds" to "creds_ctx"

Trying to understand this code it's important for me to name variables
indicating their use: A netlogon_creds_cli_context is a context with access to
credentials, it's not the credentials itself.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
