Merge pull request #376 from rackerjoe/v4.2.2rc

Merged automatically by jenkins after successful build test
Add libvirt_cpu_mode/libvirt_cpu_model attributes
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