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;

Change-Id: I2987d4ac0480af48fcccc36a562ac7898e941dc7
Initial Functional Tests

Add functional tests for fuel_agent

Change-Id: Ic6739ff9c20547c5d9ab2b4fe9dec45bd5742db9
Fix http_driver

Change-Id: I161426be8ce909dfe78e32d9d29ed20c5ed6a5d1