utils: Fix untimeouted request in _ReadAsJson

This could cause slapos request to hang forever while fetching the
json schema of an SR, and in particular it could cause buildout to
hang forever in instance processing when requesting a sub-instance.

See merge request !575
2 jobs for master in 0 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

00:17:12

failed SlapOS.Eggs.UnitTest-Master.Python3

00:19:15