Clean up db.sqla.api.instance_* methods

Remove session parameter from public db methods:
instance_get_all_by_filters
instance_metadata_get
instance_metadata_update
instance_system_metadata_get
instance_system_metadata_update

They were never called with session parameter and they
are fully covered by tests so we are able just to remove
session parameter.

Clean up instance_metadata_get and instance_system_metadata_get
Convert rows to dict in one line instead of 4.

blueprint db-session-cleanup

Change-Id: Ib2f50cc4b5a7a2e283d891141a88c7b1ff9f7d43
Merge "Clean up db.sqla.api.instance_* methods"