Use a pool of threads to valid all remote ISOs in parallel

That way we can reduce processing time in 8s.

Without threads the requests takes 15s to complete:
GET http://localhost:8000/config/distros    200 OK      10.64s

And with the pool of threads it takes 2s:
GET http://localhost:8000/config/distros    200 OK      2.01s

Signed-off-by: Aline Manera <alinefm@br.ibm.com>