Add check definition interval ms to lrp 185036553

[jrussett]

What is this change about?

Add interval_ms param for health check definitions
This change allows the interval for the health checks to be defined and
configured in the check definition of the LRP. Previously, this value was
always filled in with a default when the health checks were created, now
the checks can define their own interval that can be used when actually
creating the checks (in the executor).

Checks are now of the form:

"check_definition": {
    "checks": [
      {
        "http_check": {
          "port": 8080,
          "path": "/health",
-         "request_timeout_ms": 10000
+         "request_timeout_ms": 10000,
+         "interval_ms": 123
        },
      }
    ],
    "readiness_checks": [
      {
        "tcp_check": {
          "port": 8080,
-         "connect_timeout_ms": 10000
+         "connect_timeout_ms": 10000,
+         "interval_ms": 123
        },
      }
    ],
    "log_source": ""
  },

Please provide any contextual information.

Associated executor PR: cloudfoundry/executor#80

Check json example adapted from our design document (Publicly Accessible):
https://docs.google.com/document/d/1AS8GzKg3pdAv56nySQTgF4NaqqzrBwmztkL2_8ahKa4/edit?usp=sharing

#185036553

Tag your pair, your PM, and/or team!

@MarcPaquette