This change updates the README for the barbican_tempest_plugin,
removing dead links and default boilerplate text and replacing it
with basic information about the plugin.

Change-Id: I7aaaad934db89e268bd54a8d28d0e77624531022
Depends-On: I3249fb879f519da63129e0ec5117377a21094974
Implements: bp tempest-plugin