Lack of instance_type setting should not cause critical failure

[WesWWagner]

This code introduces a regression where people using already existing hosts files receive a critical failure because instance_type is not set. Default values should have been added here as an option. (Blame: @tmgstevens )

line: "name: Redpanda{{ '+SASL' if sasl_enabled | default(False) | bool == True }}{{ '+TLS' if tls_enabled | default(False)|bool == True }}+{{ groups['redpanda'] | length }}x{{ instance_type }}"