Use another `execute` wrapper + tiny fixes

* Ditch oslo's `execute` wrapper from utils.py;
* Fix first partition beginning;
* Use proper arguments order for `mdadm`;
* Fix unicode string handling for fs_utils;
* Add sleep to execute;
* Fix nailgun image data fields;

Partially implements: blueprint image-based-provisioning
Change-Id: I2987d4ac0480af48fcccc36a562ac7898e941dc7
Merge "Use another `execute` wrapper + tiny fixes"