- Don't change the instance type of the dask workers - so no
  mixed instances. This is a straight port of what we have now
  to just use spot instances.
- x1 spot instances can't be created due to quota issues,
  so we remove them from the pool.

Fixes 2i2c-org#490

For spot instances, `instanceType` can not be set on the
nodePool object, or eksctl complains. Before this commit,
we were still just specifying 'instanceType', but magically
converting it to the appropriate incantation for spot instances.
This leads to confusion, and missing features (for example,
the ability to specify multiple instanceTypes).

This commit switches us to using just plain eksctl schema,
at the cost of a slightly more verbose config