Document local.conf settings for ceilometer

Describe the various settings that can be made to control the
behavior of Ceilometer. Doing in lib/ceilometer for now. When there
is more something under doc may be warranted.

Change-Id: I9cd47085a5e91bae0a88f6e26f6c8bdc86c02978
Merge "Document local.conf settings for ceilometer"