vyos.utils: T5749: fix get_vrf_members() call to iproute2

The iproute2 master argument is used for both a VRF and a bridge device. Using
this in the VRF context would retrieve and report back the wrong interfaces:

Old implementation:
===================
>>> from vyos.utils.network import get_vrf_members
>>> get_vrf_members('br1')
['eth1', 'eth2', 'vxlan1']

>>> get_vrf_members('black')
['br1.3002', 'br1.4000', 'pim6reg10200']

The new implementation:
=======================
>>> from vyos.utils.network import get_vrf_members
>>> get_vrf_members('br1')
[]
>>> get_vrf_members('black')
['br1.3002', 'br1.4000', 'pim6reg10200']

python/vyos/utils/network.py

@@ -61,14 +61,17 @@ def get_vrf_members(vrf: str) -> list:
     """
     import json
     from vyos.utils.process import cmd
-    if not interface_exists(vrf):
-        raise ValueError(f'VRF "{vrf}" does not exist!')
-    output = cmd(f'ip --json --brief link show master {vrf}')
-    answer = json.loads(output)
     interfaces = []
-    for data in answer:
-        if 'ifname' in data:
-            interfaces.append(data.get('ifname'))
+    try:
+        if not interface_exists(vrf):
+            raise ValueError(f'VRF "{vrf}" does not exist!')
+        output = cmd(f'ip --json --brief link show vrf {vrf}')
+        answer = json.loads(output)
+        for data in answer:
+            if 'ifname' in data:
+                interfaces.append(data.get('ifname'))
+    except:
+        pass
     return interfaces
 
 def get_interface_vrf(interface):