Update git submodules

* Update docs/submodules/fuel from branch 'stable/danube'
  - Double allocated memory of fuel master node

    Rise up allocated memory of fuel master node for virtual envs in order
    to avoid swapping and improve performance of overall deployment process.

    Change-Id: I04b020b628a9c84f934a9d17e8be98d34fe2a085
    Signed-off-by: Michael Polenchuk <mpolenchuk@mirantis.com>
    (cherry picked from commit c382da389e0dcffe48c428695f4b29c1c6151bd4)

diff --git a/docs/submodules/fuel b/docs/submodules/fuel
index 68898be..937d9d8 160000 (submodule)
--- a/docs/submodules/fuel
+++ b/docs/submodules/fuel
@@ -1 +1 @@
-Subproject commit 68898be998129ab6793228000f3f4528783359bf
+Subproject commit 937d9d8cf17739f670dcf40528bdc59d70a3349b