mgmt/module.py - use require_running

The "mgmt" module requires httpserver, but it's not enough that httpserver's
files be installed in the image - it also needs to be running.

Without this patch, a straight "make" (using images/default.py which uses
the "mgmt" module) produces an image without httpserver running. This patch
fixes it.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@iki.fi>