Use existing declarations for controlled vocabularies, rather than free strings in Heron metadata client

[kjsanger]

npg_pipeline::product::heron::upload::library line 14, package-level variable
our $NEB_ULTRA = q(NEB Ultra II);
in bin/npg_heron_metadata_upload, line 160, assignment of default
$sequencing_kit ||= 'NEB Ultra II';
To minimise repetition and guard against typos in further edits, could be
$sequencing_kit ||= $npg_pipeline::product::heron::upload::library::NEB_ULTRA;

Separate package with constants, which are private variable that are exported on demand or for a config file?