software/seleniumserver: add a promise to wait for all nodes to register

We are sometimes seeing test failures when nodes can not register before the
beginning of the test, this promise make sure the grid is ready to use.
7 jobs for fix/kvm-python3
Status Job ID Name Coverage
  External
passed SlapOS.SlapRunner.ResilienceTest-Master

00:29:13

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:27:13

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

04:14:21

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

02:12:19

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

05:50:20

passed SlapOS.SlapRunner.ResilienceTest-Master

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5