minimise ceilometer memory usage

according to memory_profiler, ceilometer collector and api uses
~40-45mb and ~66mb of memory respectively. importing the migration
module from oslo.db is ~5mb memory (8-15% of total memory usage).

this patch moves import so it imports the module only for migration
as it isn't needed for normal collector/api use.

Closes-Bug: #1369602
Change-Id: Ic5b116c88c40ea0b3a66694369cf6d76b9635c54
Merge "minimise ceilometer memory usage"