Allow to have different DB for alarm and metering

This changes splits the internal DB API in two parts, alarm and metering.
It also adds a new configuration options to set a different database
for alarm than the 'metering' one.

Example:

 [database]
 connection = mongodb://localhost/ceilometer
 alarm_connection = mysql://localhost/ceilometer

This changes don't take care of the drivers storaged.
They will be splitted in later changes
The ceilometer.storage.ConnectionProxy object ensure that only alarm methods
are available when the namespace is the alarm one, same for the metering
namespace.

Partial implements blueprint dedicated-alarm-database

Change-Id: I1e118e4adc59ca5cd3781dbb18865818c3cc2300