Rename private functions in db.sqla.api

instance_get_all_by_filters() called 3 helper functions: exact_filter,
regex_filter and tag_filter. None of these functions had any other
callers.

This change prefixes them with _ to identify them as private.
Additionally, it removes the model arguments to the functions as these
always relate to an Instance and/or its InstanceMetadata.
Consequently, the functions are renamed to identify them as specific
to an Instance query. exact_filter is moved from the top of the module
to be next to the other 2.

Change-Id: I671315122438022b13d6b315b577ef2a5cf42545
Merge "Rename private functions in db.sqla.api"