VOL-5144 - Create beta-release branch for bbsim

VERSION
-------
  o Bump version string for release.

.gitreview
----------
  o Assign defaultbranch=voltha-2.12 for rebase and merge.

Change-Id: Ic7efdc04046a8e331352074ec6d62e66240f22a2
diff --git a/.gitreview b/.gitreview
index abf7877..d681136 100644
--- a/.gitreview
+++ b/.gitreview
@@ -3,4 +3,4 @@
 port=29418
 project=bbsim.git
 defaultremote=origin
-
+defaultbranch=voltha-2.12
diff --git a/VERSION b/VERSION
index 04cc999..545fd57 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-1.15.6
+1.15.7