title: SoftLayer API: Does VSI flavor based order support specifying image_id
body: <p>We want to programatically order VSI using the flavor (for example. Balanced type),  however instead of using the standard os_code, we want the VSI to be created from a public image template (ie.  CentOS7-ChangeStable).   From the following doc it seems to be possible.</p>
  
  <p><a href="http://softlayer-python.readthedocs.io/en/latest/_modules/SoftLayer/managers/vs.html" rel="nofollow noreferrer">http://softlayer-python.readthedocs.io/en/latest/_modules/SoftLayer/managers/vs.html</a></p>
  
  <p>However I tried but got the following error:</p>
  
  <pre><code>SoftLayer.exceptions.SoftLayerAPIError: SoftLayerAPIError(SoftLayer_Exception_InvalidValue): Invalid value provided for 'blockDevices'. Block devices may not be provided when using an image template.
  </code></pre>
  
  <p>Using slcli is failing as well with a different error:</p>
  
  <pre><code># slcli vs create --hostname testvsi --domain vmonic.local --flavor BL2_4X8X100 --image 1cc8be72-f230-4ab9-b4b2-329c3e747853 --datacenter tok02 --private
  This action will incur charges on your account. Continue? [y/N]: y
  SoftLayerAPIError(SoftLayer_Exception_Public): Order is missing the following category: Operating System.
  </code></pre>
  
  <p>Please advice whether using "image_id" with "flavor" is supported in SL API / python API.   Thanks!</p>