Add diskimage-builder to the list of libraries

Add diskimage-builder to the list of libraries so it can be installed
from source to be tested in gate.

Change-Id: I6cefac1eb4ebf0196a6b4d4bfc038c00921f0d70
diff --git a/stackrc b/stackrc
index 16621f1..a42638c 100644
--- a/stackrc
+++ b/stackrc
@@ -473,6 +473,10 @@
 # this doesn't exist in a lib file, so set it here
 GITDIR["ironic-lib"]=$DEST/ironic-lib
 
+# diskimage-builder tool
+GITREPO["diskimage-builder"]=${DIB_REPO:-${GIT_BASE}/openstack/diskimage-builder.git}
+GITBRANCH["diskimage-builder"]=${DIB_BRANCH:-master}
+GITDIR["diskimage-builder"]=$DEST/diskimage-builder
 
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