Update git submodules

* Update docs/submodules/fuel from branch 'master'
  - requirements_rpm: Add epel-release, nbd

    qemu-nbd requires the 'nbd' kernel module, which is not available
    by default on CentOS 7, but is available from EPEL repo.

    Change-Id: I3e8f722d31a97293b077115499a97c93a4751917
    Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>

diff --git a/docs/submodules/fuel b/docs/submodules/fuel
index f37103b..52e73b6 160000 (submodule)
--- a/docs/submodules/fuel
+++ b/docs/submodules/fuel
@@ -1 +1 @@
-Subproject commit f37103b4d60f057f653e0e5e8b7c6c3023a95b2d
+Subproject commit 52e73b648550fb8f3549af690d579a503a982ecf