Make service health checks configurable

[mmdriley]

When a service specifies protocol: "http" for one of its ports, we should allow customization of at least:

• the health check URL
• the set of healthy responses

Full set of ELB health check options: https://docs.aws.amazon.com/elasticloadbalancing/latest/application/target-group-health-checks.html

[lukehoban]

Related to this - we also had a request to expose configuration of healthCheckGracePeriodSeconds as part of overall health check configuration on cloud.Service (or possibly initially just even cloudaws.Service).

[lukehoban]

We are adding AWS-specific libraries for working with ECS Services as part of pulumi/pulumi-aws-infra#53. In the near term, those will offer greater flexibility to control details like this for users who are working with AWS. We'll continue to think about how to add cross-cloud support for this as part of the cloud package.