Add tools/tempest_coverage.py script.

This adds support for testing tempest's coverage on nova.
tools/tempest_coverage.py will enable coverage reporting in
nova to be started and stopped. It also performs coverage report
generation.

When it is enabled from run_tests with '-c' or '--nova_coverage'
a single text report file will be generated per nova service.

Implements: blueprint tempest-coverage-reporting
Change-Id: I00a52fb013c5a7a66a2317dbd5359a22d35bdb29
Signed-off-by: Matthew Treinish <treinish@linux.vnet.ibm.com>
Merge "Add tools/tempest_coverage.py script."