Prevent nova services' coverage data from combining into nova-api's

Now if combining a coverage report once, nova services' coverage
data are combined into nova-api's.
This renames nova-api coverage datafile to .nova-coverage.api and
creates a new coverage instance to combine when reporting.

Fixes bug: 1152475

Change-Id: I90b6535d490cf0123a127f8752986edd782ef37e
Merge "Prevent nova services' coverage data from combining into nova-api's"