Fix retrieving vapp edge public ip

Running vagrant up always showed the message `Removing NAT rules on [SS-EDGE] for IP [10.100.50.4].` and `Creating NAT rules on [SS-EDGE] for IP [10.100.50.4].`.
I found out that in `power_on.rb` the call to `get_vapp_edge_public_ip(vapp_id)` did not return anything before starting the vm. So I moved the code after the `poweron_vm()` call, then the `test_id` is filled and the edge gateway is not removed and created on every `vagrant up`.