switch to use ceilometer specific connections

in Juno/Kilo, we added ceilometer purpose specific connections for:
metering, alarms, event. Rather than piggyback off oslo.db's
connection option (which gives misleading sql help message), we
should use Ceilometer specific connections.

Change-Id: I7703b73708a5807fb8de89fbb828f06b488acf69
Merge "switch to use ceilometer specific connections"