Allow to add custom core plugins in release build
Allow to add custom core plugins in release build

Add a new CUSTOM list in the plugins BUCK config, which is by
default empty. This allows a cleaner patch when adding custom
plugins to be included as 'core' in the release build of a
forked Gerrit.

Change-Id: I12783a8222550983e65bf3bbcd8aef08cdcc9bed

MerritCR/merrit,gerrit-review/gerrit,gerrit-review/gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,gerrit-review/gerrit,MerritCR/merrit,MerritCR/merrit,GerritCodeReview/gerrit,MerritCR/merrit,joshuawilson/merrit,GerritCodeReview/gerrit,gerrit-review/gerrit,MerritCR/merrit,MerritCR/merrit,qtproject/qtqa-gerrit,WANdisco/gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,joshuawilson/merrit,WANdisco/gerrit,MerritCR/merrit,GerritCodeReview/gerrit,joshuawilson/merrit,WANdisco/gerrit,joshuawilson/merrit,gerrit-review/gerrit,GerritCodeReview/gerrit,MerritCR/merrit,GerritCodeReview/gerrit,joshuawilson/merrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,joshuawilson/merrit,gerrit-review/gerrit,joshuawilson/merrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,gerrit-review/gerrit,joshuawilson/merrit,GerritCodeReview/gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit