Bump up mgmt submodule head

commit d4b8635f7c1250c24af79ecf1d4f6b6de80544bc
Author: Pekka Enberg <penberg@cloudius-systems.com>

    Remove dependency to cloud-init

commit 5f96409fae88bbbad9536f4c14894318fd8d3d11
Author: Nehal J Wani <nehaljw.kkd1@gmail.com>

    Shell: Allow multiple files as arguments to command cat

commit 757156addb9fff0deadab584398e27deff0d527a
Author: Pekka Enberg <penberg@cloudius-systems.com>

    Remove the obsolete example project

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
httpserver: Fix broken test cases

Broken by commit 9ce9c37 ("httpserver: Copy-edit operating system API
documentation")...

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>