Changes

esim param_source: add is_abstract flag

Allow omitting some ParamSource subclassed from
ParamSource.get_all_implementations().

My previous attempts at automagically detecting abstract classes failed
conceptually, and the easiest, most explicit way is already used in
ConfigurableParameter: add an is_abstract flag.

Prep for Ie7171c152a7b478736f8825050305606b5af5735

Change-Id: Icfccdd0a8ecb5e0e9d22afa490d73c9f1849a64c