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