Enable block_migration_flag

Cherry-picking commits from
https://github.com/gapinc/nova/commit/ee57e7b0dc2066bf81fcaf3f3d4bff141cb82591
add a flag for config_drive_format, default to iso9660
Make scheduler_host_subset_size configurable.

Default value 1 is nova default.
Merge pull request #381 from rackerjoe/gap-block-migration

Merged automatically by jenkins after successful build test
Merge pull request #383 from rackerjoe/gap-config-drive-format

Merged automatically by jenkins after successful build test
Merge pull request #384 from gapinc/scheduler_host_subset_size

Merged automatically by jenkins after successful build test