Gui 1231 cache VPC support for scaling group wizard and detail page

https://eucalyptus.atlassian.net/browse/GUI-1231
https://eucalyptus.atlassian.net/browse/GUI-1232

This class is used to vpc_subnet value, which is not required if no VPC is selected. In such case, it was seen that the create scaling group form was causing the validation error if the VPC subnet value was missing; it appears that the wtform validation expects the parameter value to be matched with one of the values in the select options, and the preset select options for VPC subnet do not include '', thus causing the validation error when '' is returned for vpc_subnet. For this reason, the validation was disabled on this parameter.