[ipintutil]Handle exception in show ip interfaces command (sonic-net#…

…3182)

### What I did
Handle exception in show ip interfaces command when executed during config reload. Sometimes during config reload the interfaces are removed and if show ip interfaces was executed during this time we may get the below traceback. The interface would exist when the call multi_asic_get_ip_intf_from_ns was made but would have been removed in the subsequent for loop which tries to get ip interface data for each interface

```
show ip interfaces

Traceback (most recent call last):
  File "/usr/local/bin/ipintutil", line 276, in

     main()
  File "/usr/local/bin/ipintutil", line 269, in main
    ip_intfs = get_ip_intfs(af, namespace, display)
  File "/usr/local/bin/ipintutil", line 232, in get_ip_intfs
    ip_intfs_in_ns = get_ip_intfs_in_namespace(af, namespace, display)
  File "/usr/local/bin/ipintutil", line 153, in get_ip_intfs_in_namespace
    ipaddresses = multi_asic_util.multi_asic_get_ip_intf_addr_from_ns(namespace, iface)
  File "/usr/local/lib/python3.9/dist-packages/utilities_common/multi_asic.py", line 186, in multi_asic_get_ip_intf_addr_from_ns

     ipaddresses = netifaces.ifaddresses(iface)
ValueError: You must specify a valid interface name.
```

#### How I did it
Adding try exception block so that if an interface is not present, it would be skipped.

#### How to verify it
Running show ip interface command and performing config reload in parallel

scripts/ipintutil

@@ -29,6 +29,7 @@ try:
             mock_tables.dbconnector.load_namespace_config()
         else:
             import mock_tables.mock_single_asic
+            mock_tables.mock_single_asic.add_unknown_intf=True
 except KeyError:
     pass
 
@@ -150,7 +151,10 @@ def get_ip_intfs_in_namespace(af, namespace, display):
         ip_intf_attr = []
         if namespace != constants.DEFAULT_NAMESPACE and skip_ip_intf_display(iface, display):
             continue
-        ipaddresses = multi_asic_util.multi_asic_get_ip_intf_addr_from_ns(namespace, iface)
+        try:
+            ipaddresses = multi_asic_util.multi_asic_get_ip_intf_addr_from_ns(namespace, iface)
+        except ValueError:
+            continue
         if af in ipaddresses:
             ifaddresses = []
             bgp_neighs = {}

tests/mock_tables/mock_single_asic.py

@@ -4,6 +4,8 @@
 from sonic_py_common import multi_asic
 from utilities_common import multi_asic as multi_asic_util
 
+add_unknown_intf=False
+
 mock_intf_table = {
     '': {
         'eth0': {
@@ -60,6 +62,8 @@ def mock_single_asic_get_ip_intf_from_ns(namespace):
     interfaces = []
     try:
         interfaces = list(mock_intf_table[namespace].keys())
+        if add_unknown_intf:
+            interfaces.append("unknownintf")
     except KeyError:
         pass
     return interfaces
@@ -70,7 +74,8 @@ def mock_single_asic_get_ip_intf_addr_from_ns(namespace, iface):
     try:
         ipaddresses = mock_intf_table[namespace][iface]
     except KeyError:
-        pass
+        if add_unknown_intf:
+            raise ValueError("Unknow interface")
     return ipaddresses