Add check definition interval ms to lrp 185036553 (#68)

* 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 definiton 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:
```diff
"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": ""
  },
```

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

Signed-off-by: Marc Paquette <[email protected]>

[#185036553](https://www.pivotaltracker.com/story/show/185036553)

Signed-off-by: Josh Russett <[email protected]>
Signed-off-by: Marc Paquette <[email protected]>

* Do not modify actual_lrp

We don't need to touch the actual_lrp at all for the interval_ms changes. This
file got updated when we were generating new protobuf stuff for the
changes to the desired_lrp.

This will eventually get updated by the dynamic tracing PRs, so we
aren't missing anything.

[#185036553](https://www.pivotaltracker.com/story/show/185036553)

---------

Signed-off-by: Josh Russett <[email protected]>
Signed-off-by: Marc Paquette <[email protected]>

Author: jrussett

models/check_definition.pb.go

@@ -21,10 +21,12 @@ message Check {
 message TCPCheck {
   uint32 port = 1 [(gogoproto.jsontag) = "port"];
   uint64 connect_timeout_ms = 2;
+  uint64 interval_ms = 3;
 }
 
 message HTTPCheck {
   uint32 port = 1 [(gogoproto.jsontag) = "port"];
   uint64 request_timeout_ms = 2;
   string path = 3 [(gogoproto.jsontag) = "path"];
+  uint64 interval_ms = 4;
 }