[DB] Fix labels setting in update_run

[Hedingber]

We started getting this when calling the function:

sqlalchemy.exc.IntegrityError: (raised as a result of Query-invoked autoflush; consider using a session.no_autoflush block if this flush is occurring prematurely)
(pymysql.err.IntegrityError) (1062, "Duplicate entry 'v3io_user-115' for key '_runs_labels_uc'")
[SQL: INSERT INTO runs_labels (name, value, parent) VALUES (%(name)s, %(value)s, %(parent)s)]

It's happening because we were creating new label objects instead of using the existing ones
Fixes: https://jira.iguazeng.com/browse/ML-1492