Rewrites the serveapp method into a fixture

As a fixture we will no longer have to remember to kill the server when
you are done with it. Additionally we are able to use the fixture as a
context manager.

Change-Id: If283e12cde362d3eadc96318b5cf6799a813c14a
Partial-Bug: #1247443
Add memcache options to sample config

The memcache options from keystone.common.config weren't included
in the sample config file (keystone.conf.sample).

Having these options in the sample config file makes it easier to
update them when using the memcache token backend.

Change-Id: I49b5c928bcc7f170f049277c7dad0bae58b1504e
Merge "Add memcache options to sample config"
Merge "Rewrites the serveapp method into a fixture"