VNET context default gateway is not checked if valid

Let's suppose that a Virtual network has the following default gateway

```
GATEWAY=" "
```

Being the Gateway a simple space an invalid network configuration is rendered

The solution has to do with comparing the gateway with the following bash style regex:

```
^(([1-9]?[0-9]|1[0-9][0-9]|2([0-4][0-9]|5[0-5]))\.){3}([1-9]?[0-9]|1[0-9][0-9]|2([0-4][0-9]|5
[  0-5]))$
```