Covert session reuse job to Normal if the number of files is less than a configurable threshold

there are examples of ASO jobs submitted as Session Reuse but containing only 1 transfer

https://fts3.cern.ch:8449/fts3/ftsmon/#/job/ea210b16-3de1-11e9-82ce-02163e018c08

this is quite unwanted.

we should add a new var in order to set a min number of files in a session reuse job( for instance 5)

and convert the job Normal one if the files are under the threshold