AD: use krb5_keytab for subdomain initialization
During the initialization of AD subdomains parameters like the SASL auth
id are determined. Since subdomains use a default set of the AD specific
configuration options the default keytab will be used. If krb5_keytab is
set in sssd.conf for the AD domain this keytab should be used for the
subdomains (domains of the same AD forest) as well.
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>