Allow health and liveness checks per-process type

[slack]

Relates to #879

After health checks are moved routeable process types by default, we should support setting process-type specific health checks.

deis healthcheck:{list,set,unset} should take a new argument --type=<proc type>

The list option should either enumerate checks for all process types by default or take an --all flag.

[darkhelmet]

So I assume you'd be able to provide a readiness check for each process type? Our use case is we want the default for cmd (webapp) and then an exec readiness check for sidekiq

[slack]

Exactly, that type of separation is what we are shooting for!