Ceilometer installation fixes

- MongoDB must be configured to accept connections on the public
  interface of the controller on (Open)SUSE and Debian/Ubuntu
- Fix wrong section name for the metering_key setting

Change-Id: I98a0278c08d40faa6f5f0a8c44f28bd17b274b32
backport: havana
(cherry picked from commit 65f4b44b366bc469df053ce5bc1fa91c5d2a208f)
Merge "Ceilometer installation fixes" into stable/havana