authenticate_target can't use _get_auth_attr
authenticate_target can't use _get_auth_attr

Because it doesn't return a string. Open-code support for it, and also
handle it being absent for TPG.authenticate_target.

Signed-off-by: Andy Grover <b7d524d2f5cc5aebadb6b92b08d3ab26911cde33@redhat.com>

mikenawrocki/rtslib-fb,cvubrugier/rtslib-fb,agrover/rtslib-fb,mikenawrocki/rtslib-fb