correctly parsing complete ipv6 vnet info (sonic-net#2827)

cisco sonic router will hit below error message from route_check
str3-8101-02 ERR monit[519]: 'routeCheck' status failed (255) –
Failure results: #12 "Unaccounted_ROUTE_ENTRY_TABLE_entries": #12 "fddd:a150:a0::a66:1/128",#12 "fddd:a150:a0::a67:1/128",#12 "fddd:a150:a0::a71:1/128",#12 "fddd:a150:a0::a78:1/128",#12 "fddd:a150:a0::a80:1/128"#12
#12#012Failed. Look at reported mismatches above#012add: []#012del: []

the parser will only cut the chars before : regardless it is the IPv6 format.
For instance,
route_check: "fddd:a150:a0::a21:1/128
Will be parsed as “fddd" ONLY
Vnet_v6_in_v4-0: : a150:a0::a21:1/128"
solution:
fix the parsing scheme on route check when APPL_DB refers IPv6 format.

scripts/route_check.py

@@ -436,7 +436,7 @@ def filter_out_vnet_routes(routes):
     vnet_routes = []
 
     for vnet_route_db_key in vnet_routes_db_keys:
-        vnet_route_attrs = vnet_route_db_key.split(':')
+        vnet_route_attrs = vnet_route_db_key.split(':', 1)
         vnet_name = vnet_route_attrs[0]
         vnet_route = vnet_route_attrs[1]
         vnet_routes.append(vnet_route)