Description of problem:
when we use API to register a new host into a hostgroup, without specifying the OS, the request is accepted instead of being rejected  due to lack of needed params and hosts are registered with unexpected defaults.

Version-Release number of selected component (if applicable):
Satellite 6.1.6

How reproducible:
100%

Steps to Reproduce:
1) if we create a new hostgroup without specifying architecture or OS (we can do the same via the webui even when there is marked a * for mandatory fields for arch and os):
# hammer hostgroup create --name "AXE"  --organizations "V-TS"

API:
https://localhost:8443/api/v2/hostgroups?search=AXE

{
  "total": 28,
  "subtotal": 1,
  "page": 1,
  "per_page": 20,
  "search": "AXE",
  "sort": {
    "by": null,
    "order": null
  },
  "results": [{"id":44,"name":"AXE","title":"AXE","subnet_id":null,"subnet_name":null,"operatingsystem_id":null,"operatingsystem_name":null,"domain_id":null,"domain_name":null,"environment_id":null,"environment_name":null,"compute_profile_id":null,"compute_profile_name":null,"ancestry":null,"puppet_proxy_id":null,"puppet_ca_proxy_id":null,"ptable_id":null,"ptable_name":null,"medium_id":null,"medium_name":null,"architecture_id":null,"architecture_name":null,"realm_id":null,"realm_name":null,"created_at":"2016-02-11T16:56:06Z","updated_at":"2016-02-11T16:56:06Z"}]
}


2) If we specify an arch / OS / Media / partition save it, this is the output from API:

{
  "total": 28,
  "subtotal": 1,
  "page": 1,
  "per_page": 20,
  "search": "AXE",
  "sort": {
    "by": null,
    "order": null
  },
  "results": [{"id":44,"name":"AXE","title":"AXE","subnet_id":null,"subnet_name":null,"operatingsystem_id":2,"operatingsystem_name":"RHEL Server 6.7","domain_id":null,"domain_name":null,"environment_id":null,"environment_name":null,"compute_profile_id":null,"compute_profile_name":null,"ancestry":null,"puppet_proxy_id":null,"puppet_ca_proxy_id":null,"ptable_id":7,"ptable_name":"Kickstart default","medium_id":8,"medium_name":"Default_Organization/Library/Red_Hat_Server/Red_Hat_Enterprise_Linux_6_Server_Kickstart_x86_64_6_7","architecture_id":1,"architecture_name":"x86_64","realm_id":null,"realm_name":null,"created_at":"2016-02-11T16:56:06Z","updated_at":"2016-02-11T17:02:26Z"}]
}

3) If we revert the modification, by putting all fields blank, this is the result from API:

{
  "total": 28,
  "subtotal": 1,
  "page": 1,
  "per_page": 20,
  "search": "AXE",
  "sort": {
    "by": null,
    "order": null
  },
  "results": [{"id":44,"name":"AXE","title":"AXE","subnet_id":null,"subnet_name":null,"operatingsystem_id":null,"operatingsystem_name":null,"domain_id":null,"domain_name":null,"environment_id":null,"environment_name":null,"compute_profile_id":null,"compute_profile_name":null,"ancestry":null,"puppet_proxy_id":null,"puppet_ca_proxy_id":null,"ptable_id":7,"ptable_name":"Kickstart default","medium_id":8,"medium_name":"Default_Organization/Library/Red_Hat_Server/Red_Hat_Enterprise_Linux_6_Server_Kickstart_x86_64_6_7","architecture_id":null,"architecture_name":null,"realm_id":null,"realm_name":null,"created_at":"2016-02-11T16:56:06Z","updated_at":"2016-02-11T17:03:57Z"}]
}

Actual results:
By UI all appears blank, but from API all fields but Architecture are still populated .

Expected results:
The request should be rejected.

Additional info: