Document keystone_authtoken section

It's necessary to document how to configure the keystone_authtoken
section in the ceilometer.conf file so clients can authenticate
against the API server.

Fixes bug: 1177765

Change-Id: I93b45a7e65546246bbf1af795be5887e9a80c1fa
(cherry picked from commit c043106309415d7540c22f80eae9963927cd7f6c)
fix the broken ceilometer.conf.sample link

Change-Id: I37ce2b1e10019e5a77217d458113155a26a2c59f
Fixes: bug #1177332
(cherry picked from commit 1d4533e915a2d812af19d61937916b882d1f7da1)
Merge "Document keystone_authtoken section" into stable/grizzly