util: use zc.buildout.download API to download schema

This will support buildout extension to download from gitlab API with
authentication.

The code has already been refactored a bit to be able to pass a
download instance to SoftwareReleaseSchema, for now to instanciate only
one Download but in the future this might be extended to pass some
configuration to the Download instance.
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-TestRunner1.Python2

00:36:21

passed SlapOS.Eggs.UnitTest-TestRunner1.Python3

00:41:04

passed SlapOS.Eggs.UnitTest-TestRunner1.Python2

00:42:10

passed SlapOS.Eggs.UnitTest-TestRunner1.Python2

01:46:28

passed SlapOS.Eggs.UnitTest-TestRunner1.Python2

00:28:56

failed SlapOS.Eggs.UnitTest-TestRunner1.Python3

01:28:31

failed SlapOS.Eggs.UnitTest-TestRunner1.Python3

failed SlapOS.Eggs.UnitTest-TestRunner1.Python3

00:25:49

passed SlapOS.Eggs.UnitTest-TestRunner1.Python3

00:38:05