Add nullable instance health state & health_check delay field

[mattoni]

Adds new health check properties being implemented in the platform.

• On an instance, state.health.healthy is now nullable, implying the first health check has not yet been performed
• Added a delay field to the container deploy -> health_check config. This duration tells Cycle how long to wait before performing an initial health check (to give sufficient time for the container instance to start before falsely responding).
• Corrects a mistake where instance state.health is actually nullable.

[aubuchcl]

This seems correct in structure and raised any questions that came up. Approved.