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
Merge "Add explicit libkrb5-dev dependency for keystone"