disk_template param is optional in instance create

The disk_template parameter is optional during disk create (the first
enabled disk template is taken in this case), so don't require it.
Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>

src/Ganeti/OpCodes.hs

@@ -444,7 +444,7 @@ $(genOpCode "OpCode"
      , pOpportunisticLocking
      , pInstBeParams
      , pInstDisks
-     , pDiskTemplate
+     , pOptDiskTemplate
      , pFileDriver
      , pFileStorageDir
      , pInstHvParams