Implements send-data-to-ceilometer

This patch will define the sensor data collection interface and implement
a driver based on IPMI to collect sensor data and send them to Ceilometer.

The change covered by this patch:

1. Creates a new ironic.driver.base.ManagementInterface common method
*get_sensors_data* for gathering hardware sensor data.

2. Implements the interface into 'ipmitool' driver to gather data via IPMI
command call.

3. Adds periodic task to conductor which emits notification to Ceilometer by
an interval.

Spec: https://review.openstack.org/102435

Change-Id: I2c072321a662db0162c938b7d4f3b59ba07d4e08
Implements: blueprint send-data-to-ceilometer
Merge "Implements send-data-to-ceilometer"