[storm_on_demand|compute] Add 'category' attribute

Storm has three categories of instances: storm, ssd, and bare-metal:
- storm: "Generic" instances chosen by RAM amount and CPU cores
- ssd: Same as "storm" but instance storage, including boot volume, will
  be on solid-state hard drives
- bare-metal: The virtualized instance will be the only instance running
  on the specified configuration
[storm_on_demand|compute] View configs by category

Specify 'storm', 'ssd', or 'bare-metal' to view available configurations
by category