schema: Speed up schema cache
Check presence of schema in cache (and download it if necessary) on
__init__ instead of with each __getitem__ call. Prefill internal
dictionary with empty record for each command to be able to quickly
determine if requested command exist in schema or not. Rest of schema
data are read from cache on first attempt to retrive them.
https://fedorahosted.org/freeipa/ticket/6048
https://fedorahosted.org/freeipa/ticket/6069
Reviewed-By: Jan Cholasta <jcholast@redhat.com>