Update git submodules

* Update devstack from branch 'master'
  to 520e144c1b9942bb57cc0f03dec1726fd1bca065
  - Merge "Remove option for apache < 2.4"
  - Remove option for apache < 2.4

    apache 2.4 was released long time ago and is now available in recent
    operating systems.

    Change-Id: If367869e8490159f31c7d6c0207e182dd7ecb164
    Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

diff --git a/devstack b/devstack
index f755f3ffe3b024df7bdc0784b61521f83b831102..520e144c1b9942bb57cc0f03dec1726fd1bca065 160000 (submodule)
--- a/devstack
+++ b/devstack
@@ -1 +1 @@
-Subproject commit f755f3ffe3b024df7bdc0784b61521f83b831102
+Subproject commit 520e144c1b9942bb57cc0f03dec1726fd1bca065