[smartswitch] Align the smart switch config generator with the YANG model. (#17636)

- Why I did it
Align the smart switch config generator with the YANG model.

- How I did it
Change MID_PLANE_BRIDGE table field name in the generated config from address to ip_prefix.

- How to verify it
Run UT. The tests are aligned with the changes.

Signed-off-by: Oleksandr Ivantsiv <oivantsiv@nvidia.com>

src/sonic-config-engine/config_samples.py

@@ -86,7 +86,7 @@ def generate_t1_smartswitch_sample_config(data):
     data['MID_PLANE_BRIDGE'] = {
         'GLOBAL': {
             'bridge': bridge_name,
-            'address': '{}/24'.format(mpbr_address)
+            'ip_prefix': '{}/24'.format(mpbr_address)
         }
     }
 

src/sonic-config-engine/tests/sample_output/t1-smartswitch.json

@@ -554,7 +554,7 @@
     "MID_PLANE_BRIDGE": {
         "GLOBAL": {
             "bridge": "bridge_midplane",
-            "address": "169.254.200.254/24"
+            "ip_prefix": "169.254.200.254/24"
         }
     },
     "DHCP_SERVER_IPV4": {