stack/caucase: generate key / csr only once

If key or csr are already present, we should not re-run this
openssl command which generates a new key and a new CSR.
5 jobs for fix/caucase
Status Job ID Name Coverage
  External
passed SlapOS.SlapRunner.ResilienceTest-Master

00:24:05

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:30:17

running SlapOS.SoftwareReleases.IntegrationTest-Jerome-Python3

32381:42:03

running SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

32381:20:26

passed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

03:48:11