Support ceilometer in trove

Will good to have integration with Ceilometer.
Use cases:
- get stats about instances/backups usage
- get notifications about instance status changes (eg. crash, resize, etc..)
- existing events should be keep regarding contract
    https://wiki.openstack.org/wiki/Trove/trove-notifications

New event notifications should be added:
  instance.create.start|end|error
  instance.delete.start|end|error
  instance.resize.start|end|error
  instance.migrate.start|end|error
  instance.backup.start|end|error
  instance.reboot(restart?).start|end|error
  instance.fail
  backup.delete

New mgmt API routes for pollster should be added:
Instances stats:
GET {tenant_id}/mgmt/stats/instances
# returns list of all instances for all tenants
available attributes:
id
created
flavor_id
hostname
name
tenant_id
volume_size
updated

Backups stats:
GET {tenant_id}/mgmt/stats/backups
# returns list of all backups for all tenants
available attributes:
'id'
'name'
'description'
'backup_type',
'size'
'tenant_id'
'state'
'backup_timestamp'
'created'
'updated'

Events about database/users operations are not provided, cuz hard of implementation.

Ceilometer plugin should be implemented.