Fixes #16197 - Host#smart_proxies returns only the host proxies

Doing the following:
- Create 2 smart proxies, one for dhcp/dns/tft/puppetca/puppet, another
for realms. Let's keep the realms proxy off, so it's unreachable.
- Create a hostgroup A that sets domain, subnet, realm, puppet_proxy,
puppet_ca_proxy
- Create a host that uses hostgroup A, but uncheck 'inherit' and
remove the 'realm' attribute. The host is created just fine with no
realm
- When I try to rebuild the host, `HostBuildStatus#check_all_statuses`
is called. This checks the host_status (OK), templates_status(OK), and
smart_proxies_status(FAIL). The host cannot be rebuilt.

The reason `smart_proxies_status` fails is that `smart_proxy_ids`
tries to find whether Realm is set.
- If it's set, it adds the proxy to 'Host#smart_proxies`
- If it's not set, it looks in the hostgroup to see if it's set there.
However, I have overridden the option, because I don't want to set a
Realm since I don't want to boot the Realm proxy I have.

Host#smart_proxies should not look in the host.hostgroup object to
retrieve values for the proxy, but instead retrieve those from itself