allow multiple asset types to be graphable

[cburroughs]

Currently the isGraphable check looks someting like asset.isServerNode && asset.getHostnameMetaValue.isDefined. I would like to have graphs for vms/containers/some-other-non-physical server thing.

Making isGraphable a property of the asset type seemed like a reasonable course.

[byxorna]

What would the isGraphable property actually look at? Probably would be best to push this into a config so you can specify the asset types you want to be graphable, and the attributes required on them.

[cburroughs]

I was thinking it would be an attribute of the Asset Type, but now that I look it doesn't seem that Asset Type's have attributes. Maybe they should? But that seems like a larger change then the config based approach.