Added libxml2-dev package as a pre-requisite when installing tempest.
This is required so that devstack can be installed on vanilla Ubuntu systems.

Closes-Bug: 1225395

Change-Id: Id9116e00e18c23e8e6391d8aa652c04d8bdb86c3
diff --git a/files/debs/tempest b/files/debs/tempest
index f244e4e..bb09529 100644
--- a/files/debs/tempest
+++ b/files/debs/tempest
@@ -1 +1,2 @@
-libxslt1-dev
\ No newline at end of file
+libxml2-dev
+libxslt1-dev