Update git submodules

* Update docs/submodules/releng from branch 'master'
  - Leverage on $WORKSPACE in xtesting.yaml

    It also forces the right slave when pulling/removing the docker images.

    Change-Id: I36c4b0d7f046c25d51458ec6dee0da163eb3ee2d
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index f022b47..6251581 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit f022b470e2828520e7179001c97dbecb449241c2
+Subproject commit 62515818796b3b0ef9741ebdef6568cc837b124d