Update git submodules

* Update docs/submodules/fuel from branch 'stable/danube'
  - Merge "Select the fastest mirror of repos" into stable/danube
  - Select the fastest mirror of repos

    https://bugs.launchpad.net/fuel/+bug/1669751 has been resolved.

    JIRA: FUEL-261
    Change-Id: I009cf6cba8aaa7ff2ae625c5067aa4d1570e4793
    Signed-off-by: Michael Polenchuk <mpolenchuk@mirantis.com>
    (cherry picked from commit b6ba4fbf280da3a9c03ddfc58e47b52a9c266310)

diff --git a/docs/submodules/fuel b/docs/submodules/fuel
index a77f057..7593ef7 160000 (submodule)
--- a/docs/submodules/fuel
+++ b/docs/submodules/fuel
@@ -1 +1 @@
-Subproject commit a77f0573946c9b12b9665b19216202f1f740f084
+Subproject commit 7593ef7c97a509e494cea6603b507c9690fd741e