[SSSD] [PATCH] Add mandatory flag to SSSD config schema

Also add list_mandatory_options() to both SSSDService and SSSDDomain objects.

There is a new list_options_with_mandatory() function that will return a longer tuple than list_options(), including the mandatory flag directly.

list_options() will still return the shorter tuple, for backwards-compatibility.

This feature will allow authconfig (a consumer of the SSSDConfig API) to produce a simpler interface listing only the mandatory options for each provider. This should result in a less confusing GUI.