Update git submodules

* Update docs/submodules/so.git from branch 'master'
  to 21e51ae1290ac58d9f579333199098f98615f5b2
  - Merge "Missing VNF Recipe for Optical service"
  - Missing VNF Recipe for Optical service

    Added missing Recipe for orchestration of OTN services in Optical domain

    Change-Id: I515448ca6c799fbcaddbbe10fe803e47308cd8fe
    Issue-ID: SO-2773
    Signed-off-by: Srinivasan Ramanathan <srinivasan.ramanathan2@us.fujitsu.com>

diff --git a/docs/submodules/so.git b/docs/submodules/so.git
index 87c4b1b..21e51ae 160000 (submodule)
--- a/docs/submodules/so.git
+++ b/docs/submodules/so.git
@@ -1 +1 @@
-Subproject commit 87c4b1b30ef478cdcdc7a0b2a5c73223d7fd045a
+Subproject commit 21e51ae1290ac58d9f579333199098f98615f5b2