Arbitrary filter condition on meta table

[mansenfranzen]

pybatchintory fetches data items from the meta data table. Often, only a subset of data items may be relevant. Hence, an arbitrary filter condition should be applicable to subset only desired data items for full flexibility.

Ideally, the filter condition is not a raw string but can be passed to sqlalchemy to guard against sql injections. The filter condition will need to be stored in the job_config in the inventory table (see #1).