…ased on pod density

(11*maxPodsPerNode+255) instead of total memory.
See https://github.com/awslabs/amazon-eks-ami/blob/21426e27e3845319dbca92e7df32e5c4b984a1d1/files/bootstrap.sh#L163

This commit introduces a new MaxPodsPerNode property on InstanceTypeInfo and populates it
using the data from maxpods.go so that it will be available for calculating the default memory
to reserve. It removes the Memory property from InstanceTypeInfo because it is no longer needed.

This commit also updates related unit tests. It fixes some unit tests which reference instance types which no
longer exist and adds a check to make sure instance types which are expected to exist actually exist, and those
which are not expected to exist do not exist.