Enable SPAKE support using krb5.conf.d snippet
Because krb5 silently ignores unrecognized options, this is safe on
all versions. It lands upstream in krb5-1.17; in Fedora, it was added
in krb5-1.6-17.
Upstream documentation can be found in-tree at
https://github.com/krb5/krb5/blob/master/doc/admin/spake.rst
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Christian Heimes <cheimes@redhat.com>