Update git submodules

* Update docs/submodules/doctor from branch 'master'
  - create flavor for Apex installer

    As doctor functest CI runs failed in Apex installer, it indicates
    that flavor is None. See http://artifacts.opnfv.org/logs/functest/lf-pod1/2018-01-21_05-30-00/doctor-notification.log

    Change-Id: Ia7a7111a7c6b94864f2e12f30969fe6e411058af
    Signed-off-by: dongwenjuan <dong.wenjuan@zte.com.cn>

diff --git a/docs/submodules/doctor b/docs/submodules/doctor
index ffb5048..8c4ab68 160000 (submodule)
--- a/docs/submodules/doctor
+++ b/docs/submodules/doctor
@@ -1 +1 @@
-Subproject commit ffb504843611959b983614bd7b316cd3c205fd49
+Subproject commit 8c4ab68d34c0298cc5f886fe47ddd5e3154dd54d