Add support to get fabric asic namespaces list. (#11793)

Why I did it
VoQ chassis supervisor will have Fabric asics and the sub_role for fabric asics will be "Fabric".
The fabric asics namespaces are not being returned in get_all_namespaces() and is required in caclmgrd to add right cacl to allow internal docker traffic from fabric asic namespaces.
test_cacl_application fails on VoQ chassis Supervisor with the error:
Failed: Missing expected iptables rules: set(['-A INPUT -s 240.127.1.1/32 -d 240.127.1.1/32 -j ACCEPT', '-A INPUT -s 240.127.1.3/32 -d 240.127.1.1/32 -j ACCEPT', '-A INPUT -s 240.127.1.2/32 -d 240.127.1.1/32 -j ACCEPT'])

How I did it
Update get_all_namespaces to return fabric namespaces list.

How to verify it
Verified on VoQ chassis.

src/sonic-py-common/sonic_py_common/multi_asic.py

@@ -13,6 +13,7 @@
 ASIC_CONF_FILENAME = 'asic.conf'
 FRONTEND_ASIC_SUB_ROLE = 'FrontEnd'
 BACKEND_ASIC_SUB_ROLE = 'BackEnd'
+FABRIC_ASIC_SUB_ROLE = 'Fabric'
 EXTERNAL_PORT = 'Ext'
 INTERNAL_PORT = 'Int'
 PORT_CHANNEL_CFG_DB_TABLE = 'PORTCHANNEL'
@@ -208,6 +209,7 @@ def get_all_namespaces():
     """
     front_ns = []
     back_ns = []
+    fabric_ns = []
     num_asics = get_num_asics()
 
     if is_multi_asic():
@@ -222,8 +224,10 @@ def get_all_namespaces():
                 front_ns.append(namespace)
             elif metadata['localhost']['sub_role'] == BACKEND_ASIC_SUB_ROLE:
                 back_ns.append(namespace)
+            elif metadata['localhost']['sub_role'] == FABRIC_ASIC_SUB_ROLE:
+                fabric_ns.append(namespace)
 
-    return {'front_ns': front_ns, 'back_ns': back_ns}
+    return {'front_ns': front_ns, 'back_ns': back_ns, 'fabric_ns': fabric_ns}
 
 
 def get_namespace_list(namespace=None):