PROV-913 follow up enhancements

+ Relationship generator plugin: added support for per-field `value_converter`
  configuration items, which gives the name of a function that the value is passed
  through before being matched against the trigger.  This could be a `trim()` or
  a call to `caGetListItemIdno()` to match against idno instead of numeric id.
+ Added test for `value_converter`.
+ Separated out an AbstractPluginIntegrationTest abstract class, which gives some
  basic infrastructure useful for unit / integration tests on plugins, specifically
  the utility to store objects created during a test run and clean them up
  automatically.