mv_machine_learning: face recognition: allow 5 set per a label

mv_machine_learning: face recognition: allow 5 set per a label

[Issue type] New feature

Added a new feature that it allows maximum 5 set of feature vector
per a label. In case of few-shot leanring based on KNN algorthm,
Feature vector set more than 5 aren't needed.

This is a temporary solution so I will introduce a feature vector priority
based solution if NNTrainer is ready for this later.

Change-Id: I57d24f36101b354ea2e0fed8134f99a31e32307e
Signed-off-by: Inki Dae <inki.dae@samsung.com>