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

[dLobatog]

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

[tbrisker]

Looks good, will merge if [test] is green

[dLobatog]

@tbrisker They are greener than this apple 🍏

[tbrisker]

Merged as 15f2f29, thanks @dLobatog!