Don't set initial_node_count when autoscaling disabled on node pool

[taylorludwig]

Fixes #311

When autoscaling is false for a node pool only node_count should be set. initial_node_count should be empty.

Also uses a new node pool var node_count rather than using min_count to match the provider vars. #297 should be updated to include this (defaults to 1).

A node pool without autoscaling can be created with

{
    autoscaling = false
    node_count = 2 
    ...
}