Fix issue when Nova does not support DiskConfig

When DiskConfig is not support by Nova, the disk_config field is
removed from the form. This fix takes care of safely handling the
case when the field is not available.

Change-Id: I44727eb3fc44c209b3b8cf28ab89a45c7d3bcfa1
Closes-Bug: #1326406
Merge "Fix issue when Nova does not support DiskConfig"