Description of problem:

A subnet can be associated with a discovery proxy. The API documentation **does not** makes mention of this in the documentation for POST/GET/PUT /api/v2/subnets but the UI exposes this capability when creating (or editing) a new subnet. (See the relevant API documentation page at https://sat.example.com/apidoc/v2/subnets.html and search for "discovery".) However, I can POST to /api/v2/subnets with a discovery proxy ID and the entity will be created and associated to the correct capsule. GET /api/v2/subnets/:id does not return the discovery proxy information.

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

* Tested against Satellite-6.1.0 Beta

How reproducible:
100%

Steps to Reproduce:
0. Create a new subnet associated to a Discovery Proxy
1. Issue an HTTP GET request to https://sat.example.com/api/v2/subnets/:id
2. Examine the response.
3. Note that discovery proxy is not mentioned.

Actual results:

{u'boot_mode': u'DHCP',
 u'cidr': 24,
 u'created_at': u'2015-04-29T15:34:21Z',
 u'dhcp': {u'id': 1,
  u'name': u'ibm-x3250m4-06.lab.eng.rdu2.redhat.com',
  u'url': u'https://ibm-x3250m4-06.lab.eng.rdu2.redhat.com:9090'},
 u'dns': {u'id': 1,
  u'name': u'ibm-x3250m4-06.lab.eng.rdu2.redhat.com',
  u'url': u'https://ibm-x3250m4-06.lab.eng.rdu2.redhat.com:9090'},
 u'dns_primary': None,
 u'dns_secondary': None,
 u'domains': [{u'id': 1, u'name': u'lab.eng.rdu2.redhat.com'}],
 u'from': None,
 u'gateway': None,
 u'id': 7,
 u'interfaces': [],
 u'ipam': u'DHCP',
 u'locations': [{u'id': 2,
   u'name': u'Default Location',
   u'title': u'Default Location'}],
 u'mask': u'255.255.255.0',
 u'name': u'libvirt_subnet',
 u'network': u'192.168.100.0',
 u'network_address': u'192.168.100.0/24',
 u'organizations': [{u'id': 1,
   u'name': u'Default Organization',
   u'title': u'Default Organization'}],
 u'priority': None,
 u'tftp': {u'id': 1,
  u'name': u'ibm-x3250m4-06.lab.eng.rdu2.redhat.com',
  u'url': u'https://ibm-x3250m4-06.lab.eng.rdu2.redhat.com:9090'},
 u'to': None,
 u'updated_at': u'2015-04-29T15:34:21Z',
 u'vlanid': None}

Expected results:
Same as above, but with realms mentioned.