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.
3 jobs for fix/seleniumserver-capri-testnode-2 in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP2

08:40:41

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP2

10:05:30

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP2

08:22:00