spec: pod manifest: volumes: add recursive field

The recursive field is a boolean. I set the default to true, but only
with a "SHOULD" (rfc2119):

> 3. SHOULD   This word, or the adjective "RECOMMENDED", mean that there
>    may exist valid reasons in particular circumstances to ignore a
>    particular item, but the full implications must be understood and
>    carefully weighed before choosing a different course.

See discussion on:
#622 (comment)

spec/pods.md

@@ -129,7 +129,8 @@ JSON Schema for the Pod Manifest, conforming to [RFC4627](https://tools.ietf.org
             "name": "worklib",
             "kind": "host",
             "source": "/opt/tenant1/work",
-            "readOnly": true
+            "readOnly": true,
+            "recursive": true
         }
     ],
     "isolators": [
@@ -175,6 +176,7 @@ JSON Schema for the Pod Manifest, conforming to [RFC4627](https://tools.ietf.org
     * **kind** (string, required) either:
         * **empty** - creates an empty directory on the host and bind mounts it into the container. All containers in the pod share the mount, and the lifetime of the volume is equal to the lifetime of the pod (i.e. the directory on the host machine is removed when the pod's filesystem is garbage collected)
         * **host** - fulfills a mount point with a bind mount from a **source** directory on the host.
+    * **recursive** (boolean, optional, only interpreted if **kind** is "host") whether or not the volume will be mounted [recursively](http://lwn.net/Articles/690679/). When **recursive** is not specified, the executor SHOULD default to recursive. However, executors in specific run environment might prefer to default to non-recursive.
     * **source** (string, required if **kind** is "host") absolute path on host to be bind mounted under a mount point in each app's chroot.
     * **mode** (string, optional, only interpreted if **kind** is "empty", defaults to `"0755"` if unsupplied) indicates the mode permission of the empty volume.
     * **uid** (integer, optional, only interpreted if **kind** is "empty", defaults to "0" if unsupplied) indicates the user id that will own the empty volume. Note it is an integer number because each app in the pod would interpret a user name differently.