Do not wait hardcoded 30 seconds when `scw exec --gateway`

[Manfred Touron]

➜  ~  scw -D exec -w --gateway=gatewesh suspicious-hopper
DEBU[0000] curl -X GET -H "X-Auth-Token: 00000000-0000-1000-0000-000000000000" -H "Content-Type: application/json" https://api.scaleway.com/servers/f9e9bfa4-87ef-430d-aa29-813c1d0491c3
DEBU[0000] The server will be accessed using the gateway '212.47.228.0' as a SSH relay
DEBU[0000] Waiting for server to be ready
DEBU[0000] curl -X GET -H "Content-Type: application/json" -H "X-Auth-Token: 00000000-0000-1000-0000-000000000000" https://api.scaleway.com/servers/cf872fe2-bfca-4d83-a91d-248dcbe05767
INFO[0001] Server changed state to 'running'
DEBU[0001] Waiting for gateway SSH port
DEBU[0001] Waiting 30 more seconds, for SSH to be ready

The 30 secs hardcoded wait is only necessary when the target server is booting, but when running scw exec -w, the server may already be booted, so this hardcoded wait should only apply on scw run and not on scw exec