Lose weight for Ceilometer log in verbose mode

When setting verbose=True, the log level will be set to INFO, so we
just change the LOG method from info to debug in places where produce
heavily output.

If want to get debug level logs, set debug=True in ceilometer.conf.

Change-Id: I06e1f06340a2e7b782d3c7461ea86b629dadf05d
Fixes: bug 1192170
(cherry picked from commit 6cf57af498b1ef30c76d4a98c3266c9d191ae16f)
Merge "Lose weight for Ceilometer log in verbose mode" into stable/grizzly