packer: fix provision race-conditions following reboot

I found out that periodically the GCE build fails after the reboot
action (part of the shell provision)
According to packer [0] it might be caused by race conditions between
the provisions and `pause_before` should be used in this case

> Sometimes, when executing a command like reboot, the shell script
will return and Packer will start executing the next one before SSH
actually quits and the machine restarts

If it's not gonna help I'll increase 10s>20s and add the ssh_read_write_timeout

[0] https://developer.hashicorp.com/packer/docs/provisioners/shell#handling-reboots

packer/scylla.json

@@ -186,7 +186,8 @@
     {
       "destination": "/home/{{user `ssh_username`}}/",
       "source": "files/",
-      "type": "file"
+      "type": "file",
+      "pause_before": "10s"
     },
     {
       "destination": "/home/{{user `ssh_username`}}/",