Add VMs subcollection to providers

[jntullo]

Example usage

/api/providers/:id/vms

• returns the VMs on a provider

{
  "name"      : "vms",
  ...
  "resources" : [
    {"href" : "/api/providers/:p_id/vms/:vm_id"}
  ]
}

/api/providers?expand=resources,vms

• It can expand vms on all providers

{
  "name"      : "providers",
  "resources" : [
    {
      "href" : "/api/providers/:id",
      ...
      "vms"  : [
       {  "href" : "/api/providers/:id/vms/:id", "name" : "vm name" ... } 
      ]
    }
  ]
}

@miq-bot add_label enhancement
@miq-bot assign @abellotti
cc: @zhitongLBN

[miq-bot]

Checked commit jntullo@5c8eafe with ruby 2.2.6, rubocop 0.47.1, and haml-lint 0.20.0
2 files checked, 0 offenses detected
Everything looks fine. 👍

[abellotti]

Short and sweet, thanks @jntullo for this enhancement !! 👍