Adding batch-create to gnt-instance
This change is part of the integration of tools/batcher from
Ganeti 1.2 into Ganeti 2.0 core code. It has a compatible
submission interface to the version from 1.2 with these differences:
* it's integrated into Ganeti directly: gnt-instance batch-create
* iallocator is now provided by the instance specs
* log is now held within Ganeti
* no force needed
* no sleep needed
* memory and vcpus goes now into beparams
* missing stuff from the TODO (see below)
Open TODOs:
* Implement notification about creation status/interactive part (top like)
* Backup instance allocation specs?
* Document usage and spec format
Reviewed-by: iustinp