lib: List all possible well-known communities in CLI (COMMUNITY_VAL_STR)

lib: List all possible well-known communities in CLI (COMMUNITY_VAL_STR)

```
exit1-debian-9(config-route-map)# set community
  AA:NN  Community number in AA:NN format (where AA and NN are (0-65535)) or local-AS|no-advertise|no-export|internet|graceful-shutdown|accept-own-nexthop|accept-own|route-filter-translated-v4|route-filter-v4|route-filter-translated-v6|route-filter-v6|llgr-stale|no-llgr|blackhole|no-peer or additive
  none   No community attribute
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>