Doesn't respect keystoneauth's --insecure flag

[Jamie Lennox]

Keystone auth specifies --insecure as a flag that can be provided to session loading. os_client_config doesn't check this though it does test the verify flag. Because we are using register_session_argparse_arguments but not load_session_from_argparse_arguments this flag is available but not re-loaded by keystoneauth.

The correct solution here would be to use load_session_from_argparse_arguments so that os_client_config automatically gains new keystoneauth options, though this becomes difficult with all the messaging that os_client_config does. For now just make it respect insecure.