Parametrise VENDOR_USER for specifying the GPG key owner

The vendor may have different keys for different users
signing the packages.

Introduce a VENDOR_USER parameter and set by default to
'yum', the one currently used for Gerrit binary distributions.

Change-Id: Id99e5724f75950a5dd4ddad9a693505ddc915f8d
diff --git a/yum-repo/Makefile b/yum-repo/Makefile
index 6910be3..806d0c1 100644
--- a/yum-repo/Makefile
+++ b/yum-repo/Makefile
@@ -5,11 +5,12 @@
 RELEASE=3
 VENDOR=GerritForge
 VENDOR_LC=$(shell echo $(VENDOR) | tr A-Z a-z)
+VENDOR_USER=yum
 WEB_URL=http://$(VENDOR_LC).com
 MIRROR_URL=http://mirrorlist.$(VENDOR_LC).com/yum
 
 # Vendor PGP Certificate (default GerritForge)
-PGP_USER=info@$(VENDOR_LC).com
+PGP_USER=$(VENDOR_USER)@$(VENDOR_LC).com
 
 ### END OF VARIABLES THAT CAN BE OVERRIDDEN ###