config: cwd is a required default

After thorough discussion, even though a reasonable default is "/", for
platform independence, leave this up to the bundle author.

Also, by this variable being present it makes things explicit for the
runtime.

Signed-off-by: Vincent Batts <[email protected]>

Author: vbatts

config.md

@@ -69,7 +69,7 @@ The runtime MUST mount entries in the listed order.
 ## Process configuration
 
 * **`terminal`** (bool, optional) specifies whether you want a terminal attached to that process. Defaults to false.
-* **`cwd`** (string, optional) is the working directory that will be set for the executable. Defaults to "/" on linux.
+* **`cwd`** (string, required) is the working directory that will be set for the executable.
 * **`env`** (array of strings, optional) contains a list of variables that will be set in the process's environment prior to execution. Elements in the array are specified as Strings in the form "KEY=value". The left hand side must consist solely of letters, digits, and underscores `_` as outlined in [IEEE Std 1003.1-2001](http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html).
 * **`args`** (string, required) executable to launch and any flags as an array. The executable is the first element and must be available at the given path inside of the rootfs. If the executable path is not an absolute path then the search $PATH is interpreted to find the executable.