mgr/orchestrator: Add cache for Inventory and Services

mgr/orchestrator: Add cache for Inventory and Services

mgr/mgr_module: Added persistent dict
  basically a pythonic interface for a k-v store
mgr/orchestrator: Added common code to implement a cache
mgr/ssh: The cache works by manually adding and removing hosts.
  only the data is invalidated
mgr/deepsea: The cache invalidates all at once and only
  valid objects are cache

Fixes https://tracker.ceph.com/issues/39990

Signed-off-by: Sebastian Wagner <sebastian.wagner@suse.com>