Write test case for measuring Disk availability.
This task includes the .yaml for the test case and the .rst for test case description.
For documentation, use template in yardstick/docs/templates
Label for documentation OPNFV_YARDSTICK_TC017_Disk availability
Metrics: percentage of time with no disk access errors (%), disk mean-time-to-failure (MTTF), number of read/write faults (per second).