Add timeout field to hooks

So that runtimes don't lock up forever waiting on a hook to return I
added a timeout field.  This allows the author to specify the timeout in
seconds that they want the hook to run before aborting the hook and
container execution.

I think the hook is the proper place to specify a timeouot like this
because any type of overall timeout in the runtime will probably not
work for all hooks and will not be flexable enough.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

config.md

@@ -226,6 +226,7 @@ If a hook returns a non-zero exit code, then an error is logged and the remainin
         "poststart": [
             {
                 "path": "/usr/bin/notify-start"
+                "timeout": 5
             }
         ],
         "poststop": [
@@ -239,6 +240,7 @@ If a hook returns a non-zero exit code, then an error is logged and the remainin
 
 `path` is required for a hook.
 `args` and `env` are optional.
+`timeout` is the number of seconds before aborting the hook.
 The semantics are the same as `Path`, `Args` and `Env` in [golang Cmd](https://golang.org/pkg/os/exec/#Cmd).
 
 ## Annotations

specs-go/config.go

@@ -96,9 +96,10 @@ type Mount struct {
 
 // Hook specifies a command that is run at a particular event in the lifecycle of a container
 type Hook struct {
-	Path string   `json:"path"`
-	Args []string `json:"args,omitempty"`
-	Env  []string `json:"env,omitempty"`
+	Path    string   `json:"path"`
+	Args    []string `json:"args,omitempty"`
+	Env     []string `json:"env,omitempty"`
+	Timeout *int     `json:"timeout,omitempty"`
 }
 
 // Hooks for container setup and teardown