Dedupe network group to extant network reference - EUCA-6665

The database reference from network group to extant network is removed
as it duplicates the reference from extant network to network group
which can lead to an inconsistent database state (reference set on the
extant network side only)

An upgrade task is added to remove the column from network group.

EUCA-6665
Merge remote-tracking branch 'origin/dev/sjones/EUCA-6665' into testing