Add explicit libkrb5-dev dependency for keystone

Our Trusty test nodes do not include libkrb5-dev by default, and it is
needed to install some of the python libraries keystone uses (kerberos,
brought in by requests-kerberos).

Partial bug: #1334326

Change-Id: I1fdaa46c4590e69da82365dee578c1aa0ec95e80
diff --git a/files/apts/keystone b/files/apts/keystone
index 57fde80..b7218b7 100644
--- a/files/apts/keystone
+++ b/files/apts/keystone
@@ -11,3 +11,4 @@
 python-routes
 libldap2-dev
 libsasl2-dev
+libkrb5-dev