Only build 'released-to-production' branch to test content schemas
Only build 'released-to-production' branch to test content schemas

Unlike dev branches, the 'released-to-production' branch should not be
built when the branch is updated. The only reason to build it is to test
it against changes to the content schemas.

This change adds Jenkins configuration which distinguish between updates
to 'released-to-production' (in which case Jenkins should do nothing)
and builds triggered by the content schema build (in which case, run the
build as normal, but check out the correct content schema branch.

The govuk-content-schemas Jenkins config will be updated to trigger this
build, and it will provide an 'IS_SCHEMA_TEST' parameter to identify it
as a schema test.

alphagov/collections-publisher,alphagov/collections-publisher,alphagov/collections-publisher