fix passing sp 'x509cert' and 'privateKey' strings in rather than file names.
__sp is only set after check_settings succeeds, therefore keys would never be found.
fix check for sls when generating sp metadata
settings __add_default_values adds sls definition with missing url
if no sls defined which causes error in metadata building.
allow overriding certificate use definition in metadata
don't advertise encryption key in metadata
enable getting metadata signin key/cert from strings instead of files