slapos/recipe/slapconfiguration: pass empty slave list & deserialise
Two changes:
- currently, when no slaves exist, [in]valid-shared-instance-list isn't set. It makes using it quite convoluted compared to simply setting it to an empty list, which is what this MR does.
- when a slave uses json-in-xml for its config, it was previously not deserialised correctly (as is the case for a root instance's config).