T5169: Allow to set CGNAT multiple internal pools

Allow to set multiple CGNAT internal pools
```
set nat cgnat pool internal int-01 range '100.64.0.0/28'
set nat cgnat pool internal int-01 range '100.64.222.11-100.64.222.14'
```

interface-definitions/nat_cgnat.xml.in

@@ -123,6 +123,7 @@
                         <validator name="ipv4-host"/>
                         <validator name="ipv4-range"/>
                       </constraint>
+                      <multi/>
                     </properties>
                   </leafNode>
                 </children>

src/conf_mode/nat_cgnat.py

@@ -253,7 +253,7 @@ def generate(config):
         int_pool_name: str = rule_config['source']['pool']
 
         external_ranges: list = [range for range in config['pool']['external'][ext_pool_name]['range']]
-        internal_ranges: list = [config['pool']['internal'][int_pool_name]['range']]
+        internal_ranges: list = [range for range in config['pool']['internal'][int_pool_name]['range']]
         external_list_hosts_count = []
         external_list_hosts = []
         internal_list_hosts_count = []