Define BRANCH to DRY out stable branch build downloads

The branch used for plugin downloads was duplicated in
the Makefile and also left behind to stable-3.4.

Fix the problem and make sure that this problem doesn't
happen again by introducing a new BRANCH variable reused
later on in the Makefile for downloads.

Change-Id: I6e738e3eba462350bfe37ea3346b7582f0c2a096
diff --git a/fpm/Makefile b/fpm/Makefile
index d17a461..962087e 100644
--- a/fpm/Makefile
+++ b/fpm/Makefile
@@ -1,5 +1,6 @@
 VERSION=3.5.1
 WAR_VERSION=3.5.1
+BRANCH=stable-3.5
 URL=https://gerrit-releases.storage.googleapis.com/gerrit-$(WAR_VERSION).war
 CI_URL=https://gerrit-ci.gerritforge.com
 RELEASE=1
@@ -22,10 +23,10 @@
 WAR=$(BUILD_ROOT)/bin/gerrit.war
 PLUGINS_CI=https://gerrit-ci.gerritforge.com/job
 LIBS_JOBS= \
-  plugin-out-of-the-box-bazel-master-stable-3.4/lastSuccessfulBuild/artifact/bazel-bin/plugins/out-of-the-box/out-of-the-box.jar
+  plugin-out-of-the-box-bazel-master-$(BRANCH)/lastSuccessfulBuild/artifact/bazel-bin/plugins/out-of-the-box/out-of-the-box.jar
 PLUGINS_JOBS= \
-  plugin-avatars-gravatar-bazel-master-stable-3.4/lastSuccessfulBuild/artifact/bazel-bin/plugins/avatars-gravatar/avatars-gravatar.jar \
-  plugin-uploadvalidator-bazel-master-stable-3.4/lastSuccessfulBuild/artifact/bazel-bin/plugins/uploadvalidator/uploadvalidator.jar
+  plugin-avatars-gravatar-bazel-master-$(BRANCH)/lastSuccessfulBuild/artifact/bazel-bin/plugins/avatars-gravatar/avatars-gravatar.jar \
+  plugin-uploadvalidator-bazel-master-$(BRANCH)/lastSuccessfulBuild/artifact/bazel-bin/plugins/uploadvalidator/uploadvalidator.jar
 
 RPM_OUT=$(OUT)/gerrit-$(VERSION)-$(RELEASE).$(BUILD_ARCH).rpm
 DEB_OUT=$(OUT)/gerrit-$(VERSION)-$(RELEASE).$(BUILD_ARCH).deb