Add [keystone_authtoken] to ironic.conf.sample

Update the auto-generation of our sample config file by exporting a
variable to oslo.config.generator such that it will include the
keystone_authtoken section when building our config.

Change-Id: I8f0d93846cd948059ac6cbc6ec4024257e2c3f27
Closes-bug: 1271308
Merge "Add [keystone_authtoken] to ironic.conf.sample"