Say something clearer than nCPUs in the UI

[david-crespo]

We got it from the API but that's no reason to confuse people.

https://github.com/search?q=repo%3Aoxidecomputer%2Fconsole+ncpus&type=code

[charliepark]

I think the one other place we use nCPUs is in the silo create form:

We could use (COUNT) (or (count)? (Count)?) in the capacity chart. In the silo create form, maybe we don't need a sublabel?