Replacing sss_lookup_user_get_attr with dbus call.

Implementing the equivalent of
	dbus-send --print-reply --system \
		--dest=org.freedesktop.sssd.infopipe \
		/org/freedesktop/sssd/infopipe \
		org.freedesktop.sssd.infopipe.GetUserAttr \
		string:$USER string:$ATTRIBUTE
Example configuration file for lookup of email and first and last names (IPA schema).