golang: v↑ go1.12 (1.12.9 -> 1.12.17)

Going Go1.12.9 -> Go1.12.17 brings many fixes including security fixes for
net/http package:

https://golang.org/doc/devel/release.html#go1.12

Tested on helloworld SR.
6 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.SlapRunner.ResilienceTest-Master

00:25:58

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:20:47

passed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

03:21:45

passed SlapOS.SlapRunner.ResilienceTest-Master

00:26:10

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:16:27

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

01:35:21