Add reset function to nova coverage.

Now nova coverage is standardized to accumulate coverage data.
It enables user to get combined coverage data of several tests.
In addition to it, we need to restart nova services when measuring
coverage of each test because there is no function to reset data.
This adds reset coverage data API.

Fixes bug: 1164331

Change-Id: Idd16cf3849b6510e7c620ed3bf16d507ed44c328
Merge "Add reset function to nova coverage."