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 will return a 500 error if the get_all_subnets() call returns an empty list.  Also, API calls should be wrapped in a boto_error_handler block if possible