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.
Merge pull request #108 from leftclickben/PROV-913_AddValueConverterSupportAndTestsRefactor

 PROV-913 follow up enhancements
PROV-923 Fix for bundle-level AC issue in install process

There were a couple of problems here:

1) We didn't validate the bundles before exporting which meant that
bundle access control restrictions for fields that no longer existed or
were no longer valid for a certain table would be exported as well.
That is fixed in the exporter.
2) There was a caching issue in the bundle validation in the Installer.
Since the metadata elements are created just before we add those access
restrictions, they didn't show up everywhere yet. The "give me a list
of all valid bundles" function returned outdated data. I now rewrote
the way we validate bundle names in the installer which should address
this issue and also speed up the "access roles" part of the installer
by a huge amount.
3) Another (unrelated) thing was that it is actually possible that
metadata element type restrictions for table that no longer exist are
still around. I changed the exporter so that this should no longer
happen.
PROV-924 Use locale code instead of ID for search form placement labels