Improve openstack-clients element.

Add keystone to the list of installed clients. Expanding this element
allows other elements, such as nagios to consistently install the
openstack clients with out having to duplicate code or depend on the
entire services element, in this case, Keystone to get the client.

Change-Id: I71e82dbf5e7d95a063ac8755c2cfc93a18ab6fb9
Authored-by: Chris Krelle <nobodycam@gmail.com>
Merge "Improve openstack-clients element."